Since the only other posts about this are buried in threads, thought I would make a thread just for this. I currently have this FoS - well for a few more days at least it seems.
https://eu.battle.net/forums/en/wow/topic/17615664181#post-4
https://eu.battle.net/forums/en/wow/topic/17615664181?page=3#post-53
This Feat of Strength wasn’t actually intended to go live. We’d decided some time ago that it didn’t really fit in line with our philosophies regarding achievements, particularly due to the non-repeatable nature of the Class Mount questline. Once 7.2.5 is released, it will no longer be granted, and will be removed from players who had already received it.
https://eu.battle.net/forums/en/wow/topic/17615664181#post-4
I can appreciate that no-one likes having something taken away from them. To try and clarify a little bit why we came to this decision; during the course of the PTR testing feedback from the community overwhelmingly pointed out that a missable achievement was both unfun for players that missed it, and unfun for players that felt compelled to slaughter innocent red dragons to achieve it.
I do also want to mention that, while the Feat of Strength will be removed, the special voice line from Bolvar will still play if players choose to follow this particular route.
https://eu.battle.net/forums/en/wow/topic/17615664181?page=3#post-53