This post reminds me of the entire expansion of BFA when a year before the drop of BFA they announced that they would be doing away with the currency Shiny Pet Charms, and moving forward you would only be able to use Polish Pet Charms for BFA pets, stones, etc. You could, before the expansion trade in your Shiny Charms for leveling stones, etc. but after BFA you would only be able to use them in for a pets you buy in Dalaran (which you can still do) by doing stuff up to BFA like garrison pet dailies, etc.
They told us on the forums, in blue posts, it was also on Wowhead, IcyVeins, etc so there were lots of places to check that the mount was leaving.