Commercial support is support offered on a business basis, to meet business needs.
Commercial support is not compulsory, and indeed many (I'd say most) open source projects do not advertise this.
On the other hand, if the writer of the software (or skilled contributors) would like to make money doing what they are great at, and people want to pay them, ...
Closing them on sight should give a clear signal that asking in the wrong places is not appreciated. From there, it is possible to guide the users making an honest effort to look in the contributing.md file.
Not after you've been given it.
When giving you support, the supporter (the dev in this case) has essentially said "I'm happy with the payment I'm getting for this." If the payment is nothing, fine. They're happy.
They can't suddenly change their mind after giving you the support and say "hey pay me for that now" - it'd be like someone giving you a present ...
Yes and no:
You have previously agreed to pay them for their service. For example if you went to the people helping you and said "I will give you $20 to help me". Them agreeing to help you under those conditions ( for $20) is a contract between you and the person helping you, you therefore need to pay him / her.
They provide support ...
When people start abusing your issue tracker to get support, then obviously your other support channels are not accessible enough.
Make sure you have a separate support system which is both easy to find and easy to use.
When I would have a problem with your software, a text file called contributing.md would not be the first place to look. In fact I would ...
What does it mean by commercial support and is it compulsory for any open-source project to have commercial support?
No, obviously not. An open source project may be a fifty line python script you wrote to solve your own problem, that you publish for others to use. No support included; just the right to use it.
Should we have to pay for such commercial ...
On Github, you can use issue templates.
These templates are shown to users while they open an issue, which means that a simple warning like Please don't ask questions here, but use https://example.com is presented to all users, even if they didn't check the repository wiki or readme.
A business may be reluctant to use open source software due to a perceived lack of reliable support for:
On the other hand, using that open source software may offer the potential of huge savings compared to a full commercial package or writing their own equivalent from scratch.
Commercial support can ...