At least in Germany taxes aren't bound to a specific purpose, they're not allowed to be. They get thrown into a big pot and then distributed as needed. Then you'd have these OSS developers (whoever represents them) fighting against other recipients.
It would be better to have the FAANGs of the world being forced to pay into a bucket which gets distributed specifically to OSS projects.
Government grants for open source software would probably be a good thing though, now that this is mentioned. Just like the arts get public funding due to public good, open source software would make sense as well. Even better than most of these things, because by definition, open source is free for the public, whereas grants for arts and even things like research are often locked behind a paywall of sorts.
The EU already does this. I'm sympathetic, but once you have witnessed how their research grants program works, or what the results are when bureaucrats fund "art", you might not be as excited. These things have a way of becoming something different than what was originally envisioned.
It would be better to have the FAANGs of the world being forced to pay into a bucket which gets distributed specifically to OSS projects.