There's too much confusion about how the LegacyVersion and LegacySpecifier interact with their non-legacy counterparts when mixed. (See #74, #112, #275, #307, #320, and probably more)

In #12 @dstufft said :

I don't think it's something that we're going to be able to realistically deprecate anytime soon.

But that was six years ago. I think the community has been publishing/using non-legacy versions for long enough that we can remove the need for this library to support it.

I propose a deprecation period, where creating a LegacyVersion/LegacySpecifier produces a deprecation warning, followed by a removal, where this would raise an InvalidVersion/InvalidSpecifier exception instead (possibly with some additional details about them being legacy).