I've noticed that some FOSS licenses explicitly mention that they are irrevocable (as long as their conditions are met) and others are silent on this point. Does this matter? Are the licenses that don't mention it revocable at-will? Is the irrevocable term redundant? Or is it affected by other things?
Here's a few licenses that don't mention whether they are irrevocable:
Here are some that explicitly mention irrevocability:
A non-exclusive copyright license (such as most FOSS licenses) can be revoked at any time only if there was no consideration involved. The United States Federal Circuit Court of Appeal took this on in Jacobsen v. Katzer in 2008 and ruled that there is consideration exchanged in the use of FOSS by a licensee. This indicates that an FOSS license that's silent on revocation is likely revocable only for violation of it's conditions.
:))
Since it seems worth having an answer that is explicitly multijurisdictional, I refer people to the article I wrote for LWN on a talk at FOSDEM 2018, given by a trio of actual lawyers, that explicitly addressed this question. While the whole talk was excellent, and remains well worth listening to, the answer to the revocability question was:
If I understand correctly (and I may well not) partial performance is a term of art from contract law, and means that the licensee (ie, you) has done at least some of the things that the licence requires you to do in order to avail yourself of the grant (eg, retaining copyright and licence notices).
But the practical upshot is that in all those jurisdictions, once you've got yourself a copy of the code, and have done the least of things with it, your licence cannot be revoked by the licensor.
IAAL and I will tell you the only possible correct answer to this question is: it depends.
We know for sure that FOSS licenses are not the same as public domain grants, which are irrevocable. With licensed FOSS, the copyright continues to exist and is held by the author(s). The authors can still sue for infringement of the copyright. A license is a defense to infringement, but a license requires consideration, i.e., there must be a contract. (Note contracts are usually governed by STATE law). No contract, no license.
A FOSS license is an example of a unilateral contract. This is a topic that's not encountered that often in modern day legal practice. I don't even pretend to fully understand all its nuances except to say that unilateral contracts CAN generally be revoked by the offeror prior to performance by the other party - the exchange of consideration. Importantly, BOTH parties have to be giving something up. It's not enough for the offeror to get something of value (the promise of a gift is not a contract). Whether the FOSS licensor gains prestige, or a warm and fuzzy feeling, that's all well and good, but there's still no contract unless the other party has given something up too.
What that "something" must be will depend on the wording of the license AND the specific facts, so getting further into that is only hypothetical and speculative. What I do believe is clear though - and this is my personal opinion not legal advice - is that a FOSS license can be revoked as to anyone who has not already used or distributed the FOSS product. In other words, if a FOSS product has been published but only browsed a few times on Github and never actually been used or distributed by anyone else, I think the author could revoke that license going forward and anyone who could not prove that they had accepted the terms of the license and given something of value - or possibly relied on the existence of the license in anticipation of doing so - before the license is revoked would be out of luck. On the other end of the spectrum is a party who has built a business around someone else's FOSS product and complied with all terms of the license (e.g., Ubuntu or Red Hat) - almost certainly this party could not have their license yanked away. But there is a huge range in between those two extremes and no one can tell you for certain what would happen in any particular case until it comes up in real life.
I see people on the Internet argue that "everyone" just "knows" that FOSS licenses are perpetual, and therefore they are. No one thought API definitions were copyrightable until Oracle decided they were, sued Google, and an appeals court agreed with Oracle. (Hopefully the Supreme Court will be correcting that travesty soon.) So while industry understandings can be persuasive, they are by no means conclusive.
In sum, anyone who relies on open source licenses for their business should consult a professional, and, if possible, have a plan B to replace the FOSS if it ever becomes necessary to do so.
(Edit) PS - While this is an old thread, I think this topic is very timely and likely to come up in real life sometime soon. We see for example Redis and other companies, resentful of how much profit other companies are making off of their software, trying to change their licenses to put in poison pills. It's only a matter of time before a prominent FOSS publisher tries to revoke their license completely and tests the waters in court.
This has been sitting idle for a while, but I came back upon it just now and happened to notice a similarity among the organizations listed in the Q. I would have just made it a comment, but it's too long. And having converted it to an answer it got even longer for completeness. Maybe it does help you understand what's being asked about, though.
Ignoring the WTFPL which doesn't talk about revocability because they are basically a wordy way of saying "This is Public Domain".
The two organizations that don't say anything about revocability have large staffs of lawyers, who write all their legal stuff; usually for B2B licenses. The others have 0 or 1 lawyer and are mostly concerned with end-user (i.e. consumer) licenses. I understand why this distinction between them results in the different language.
That understanding comes from having worked as support staff at MIT and having talked to one of the MIT licensing lawyers about some code I wrote to do my job and my desire to distribute it with a GPL license. At one stage the MIT lawyer was all set to allow it but with one final question, he had no idea what the phrase "TO THE EXTENT PERMITTED BY APPLICABLE LAW" (which at the time only occurred once and in all caps, now there are several variants in several places) was there for. To his mind having it there meant that the reverse might be true without it. But you obviously can't grant something in the license that is not "permitted by applicable law". So what was it saying? Well, the answer (from FSF's lawyer) was that this was standard required boilerplate in licenses to consumers who are not expected to realize this exemption and have lawyers to research it for them, whereas the MIT lawyer and all the company lawyers he deals with are such lawyers. There was case law (in the consumer realm) that a license without that disclaimer can be dismissed wholesale as an illegal license. Or more accurately chopped into little bits and discarded that way. And, since I was putting it up for FTP (it was pre-web) publicly on the net meant that the consumer oriented text was the right thing.
I expect there is a quite similar story behind the correlation between the irrevocable terminology and the organizations that included it. It may well be one of those case law in the consumer realm but never in B2B licenses differences. In fact, there may be a strong correlation in licenses that have one phrase also having the other.
