"Restrictive" in the context of licenses has a specific meaning: it's the opposite of "permissive" e.g. MIT and Apache. You can't ensure your licensees don't restrict their users without placing restrictions on what they can do with their work. The users aren't the ones agreeing to the license, after all; it's the people using GPL code to whom it's restrictive.

GPL and AGPL libraries are impossible to use in vast chunks of the free software ecosystem.

I think you're looking for MPL 2.0, which is copyleft while also permitting use by the entire free software ecosystem.

It’s completely possible to combine GPL and AGPL software with, e.g., BSD- or MIT-licensed software. The resulting software is GPL- or AGP-licensed, and the users are not restricted from doing anything.

I honestly thought that this was a license scanner for actually restrictive — i.e. non-free-software — licenses until I visited the repo. It would be good to know if I accidentally use some piece of software with some source-available license.

Combine if they are independent pieces of software, yes. Whether linking (hard or soft/plugin) counts is a matter of legal uncertainty. The FSF has consistently said that a single function call into a GPL library constitutes a derivative work, but other legal experts disagree [1].

Certainly, if I maintain a repo which calls a function in a GPL library, it would be misleading for me to say that my repo is MIT-licensed. I could potentially say that my specific code bits are MIT-licensed but that (per the FSF's interpretation) if you import my library, your resulting artifacts are GPL.

That is quite hard to communicate, and it also rests on a particular interpretation of the license that is the most common one but isn't universally accepted. It also becomes tricky with distributed copyright ownership without a CLA, the way most open source projects are run. I think that this rises to the level of "impossible to use".

But I guess one specific point here is that if you download my MIT-licensed-but-links-to-GPL repo, and rip out all of the bits that link to GPL libraries, you can use my code under MIT.

Compare this to MPL 2 which has none of these issues, and very little legal uncertainty in general. There is still some uncertainty regarding what constitutes a copyrightable work, though.

[1] https://tech.popdata.org/the-gpl-license-and-linking-still-u...