Overview

Tag them as 1.21.1

Why would this feature be useful?

You can't download them on instances on mod apps like Prism that are running 1.21.1.

That being said, I really can't imagine 3rd party launchers completely prevent you from downloading mods for a different version.

Unfortuantely it just doesn't detect it if you're on 1.21.1, as shown above. There is a workaround like you said, to just manually do it, however the whole purpose of these modlaunchers is to automate updating mods. I do see your point with avoiding minor versions but to be fair, 1.21.1 is pretty much the same thing as 1.21, and neoforge seems to be treating it as bleeding edge

It's automated based on the version of Minecraft we build against, and manually editing hundreds of listings on Modrinth/CurseForge would take way too long just for that. Plus there'll almost certainly be another minor version for 1.21 coming out that actually ends up breaking things and requires us to build against that version.

That being said, I really can't imagine 3rd party launchers completely prevent you from downloading mods for a different version. And if that really is the case, you can just download the version you need from the actual site, which takes an extra 10 seconds.