This doesn't solve the problem. It just makes you aware it might be there. It doesn't keep the ecosystem clean.
The result will be that everyone compiles with that flag on so they can use the libraries they want/need. It's in fact actively worse than just allowing warnings. At least if you allow warnings that way you aren't forced to lower your own codes protection just because one of your dependencies does.
Complaining about unused import doesn't solve anything IRL, if we talking about solving something. Problems in code are complex diseases, and cleaning unused entities is like washing your hands, not like taking a cure. And hand cleanliness checks is too idiotic measure to cure brain cancer. Of course all our problems are still here in both cases, but at least with Go we will die clean for sure.
If a library is so popular that "everyone" uses the flag so they can use it, surely someone can be convinced to remove the unused import statement. If the ecosystem makes that difficult then we're talking about a totally different problem.
The Go compiler could simply include a bit in the resulting binary/library that indicates whether or not it was built with the "disable-unused-variable-error" flag. If you're using a library that was built with that flag the compiler can simply refuse to compile your code without that flag, forcing you to acknowledge the problem head-on.