On a server that progresses through patches the legacy unarmored epic mounts can be purchased up until patch 1.4
To recreate this with stonetavern timeline these mounts could be made available on the vendors until Phase II
Posted from the Council — voting happens there; discuss here.
Allow me to invert the question being asked here. 
Is there a good reason to put “legacy unarmored epic mounts” into the game … only to take them out again once 80 characters have reached Level 60? 
There’s a reason, yes … because that’s what Blizzard did (over 20 years ago).
But is it a GOOD REASON to repeat that NOW on Stonetavern? 
Phase 1 Mounts: LINK
The only notable thing that I’m seeing in that list is the fact that some Epic Mounts only require Riding (1), rather than the Riding (150) skill required for all later Epic Mounts (which is a serious cash savings for a Level 60!). However, if the Innkeeper were to “normalize” all Phase 1 Epic Mounts to require Riding (150) … is there actually a GOOD REASON to “discard the legacy unarmored epic mounts” in Phase 2?
Clean up the discrepancy between Riding skill requirements and allow the legacy unarmored epic mounts to remain on offer as cosmetic only variants to the armored epic mounts (which consume finite inventory space if you want to have both!) … and I think we’re good.
My impression of the matter is that Blizzard wanted to make the Level 60 mounts “more visually distinct” (at a glance) relative to the Level 40 mounts in order to minimize confusion in PvP contexts (as if movement speed while mounted wasn’t enough of a clue). So Blizzard did what they so often do … junk what’s working to sell the NEW™ way of doing things (same as the old way!) to their Players.
The real point of contention with the idea of the legacy unarmored epic mounts being a Phase 1 ONLY thing is that it basically rewards the “sweaty no life power leveling” of racing to 60 among the people with a head start on the server. Those first 80 (or so) people then become the ONLY ONES who are eligible to obtain the legacy unarmored epic mounts, prior to Phase 2 … then once Phase 2 starts, no one else can obtain those legacy unarmored epic mounts. That means that Stonetavern will never have more than ~80 characters with those legacy unarmored epic mounts and the means to have that privilege involved Racing To The Finish 
in order to Beat The Clock. 
Is that really something that Stonetavern ought to be setting up/supporting as a desirable objective?
I’m thinking … NO. 
Conversely, do I think that adding the legacy unarmored epic mounts AS OPTIONS for purchase with no time limit (and normalized to require Riding (150) skill) for all phases of Stonetavern sounds like a good idea?
SURE! 
If it’s “just a cosmetic option” that you have to pay for (just like with any other Level 60 Mount), I see no meaningful downsides other than for PvP identification at a glance … on a PvE server.