Ofc there is a difference between someone removing an option you once had (even if it turned out to have some bad side effects on other people) and never having had that option at all. Your suggestion of an "arbitrary" rule is not accurate for this - one might argue on detail, but afaics a clear rationale was given.difference between a technical limitation (which it was many years ago) and an arbitrary rule that is enforced by admins (in a technical way)
Since we can code to add/remove technical limitations and write the lobby protocol to add/remove non-techincal limitations, from the point of view of server devs there isn't much difference between "old versions are disallowed" and "old versions are not supported" - the extent to which old engines are present was/is just another choice we have to make.
It's the server devs who implement the code, which is why I & abma announce the change. In reality, based on the strength of feeling of Kloot as lead engine dev, we don't have any real choice; although I do suspect that Kloot is right to insist on the change.