I feel current change is fine. legacyGenerationStamp looks better and inline with similar legacy things.
Suppose a new user plays with generation stamp for the very first time, she needs to know implementation details before she is able to tell which one is legacy or deprecated. Even with current V1/V2 naming, we should not blame a new user who wonders whether a generationStampV3 version exists.
First of all, Genstamp is not user-controlled or user-exposed. ( I mean, exactly user, not the developer ).
Second, Since its legacy, its kept there just to support old blocks. Not to generate new blocks with that. New blocks will always be generated with newGenstamp itself.
So IMO, its fine. No need to be so specific on implementation details.