Maybe I'm searching with the wrong terms, but can the GNU LESSER GENERAL PUBLIC LICENSE be removed from a software project like this browser extension? The commits in question are these https://github.com/igdownloader/InstagramDownloader/commit/90f2450c4f68504720f071fdc0c78321a7b85ce7 and https://github.com/igdownloader/InstagramDownloader/commit/ffc34364ea2b3e3547923c015246c87587640052
Assuming the publisher is the sole rightsholder, yes, they are perfectly free to relicense the work; however, they can't revoke the licence on existing copies that are already out there in the world. If they're not the sole rightsholder, the consent of all other rightholders will be required to relicense. That said, they've not exactly been consistent about removing the LGPL header.
What's less clear in this case is whether the current publisher is the sole rightsholder. I can see examples (eg this and this) of others contributing to the work, but we have no way of knowing whether a CLA/CTA was required by the publisher (the former might well grant permission for such relicensing, and the latter definitely would).
If you've had a contribution accepted into the work, and you've not completed a CLA or CTA, then it seems likely to me that your contributions were made under LGPL, and your copyright may now be being infringed. If this applies, and you feel strongly about it, then consult actual lawyers and prepare for a fight. Otherwise, third-party rights are a thorny and jurisdictionally-dependent issue, so there may not be much you can do about it except stopping using the work. (Full disclosure: I wrote the last linked article, but I did not give the talk which it documents.)