They’re also all GPL due to the nature of Wordpress, even the “paid” ones, so if you can find a copy you’re also in the clear - just be careful of your sources.

i believe it would be trivial to look through, if you can obtain them in the first place.

That’s the key point. Most paid plugins you can download only after paying. And if you get your hands on a copy, that doesn’t mean you’ll get updates, too. And running old plugins can considerably harm the security of your installation.

But if you can get your hands on a fully functional demo, you usually can remove limits.