TL;DR; To get older KSM/PvP mounts, you must reach KSM or whatever the PvP equivalent is, on more than one character. Unlock the current season’s reward first, then on an alt, do it again to choose 1 reward from a previous season. KSM would give the option of older mounts and PvP would give the option of an older mount + the armor set of a season.
I was trying to think of a fair way to earn these, as it make sense they’re locked to the season. There is a way to do so while you still have to do the work to earn them. Let’s pretend we’re starting this in Season 4 of Dragonflight.
KSM Mounts
Season 4, we get whatever lizard we are going to earn for Season 4 KSM. That is the 1st requirement to unlock older mounts. If we earn KSM on an alt, we really don’t earn anything. But what if we received a token that we could then use to get one of the older mounts (maybe even Challenge Mode mounts)?
I don’t know how many seasons there were in Legion or BfA, so each had 3-4. I know SLs had 4. So that being said, there are 10-12 KSM mounts pre DF. Then you have the 2 CM mounts, so 12-14 total. If you wanted to earn all 12-14 mounts in Season 4 of DF, you would have to first get KSM for season 4 DF, then get it another 12-14 times as in on new characters.
But this wouldn’t be a one-season deal. Once the season is over, the older mount would be added to the list. You still have to do the work, and you aren’t getting anything over someone who got it then. M+ difficulty has been roughly the same over the years.
PvP Mounts
I don’t exactly know how they work, but I know you have to have a rating to get Gladiator or whatever. So if rating is similar to IO, same rules. Get it done on 1 character, then repeat it on more to get access to a mount of your choice plus a PvP set of that season (for that class). I don’t know if the PvP gear is lost to time, so just in case.
As long as you are doing the work at content, there’s no reason why you can’t get the older stuff. You didn’t earn it then, so you have to earn it now, but just as my KSM idea, you have to unlock the current season first before you can get the older season’s rewards.
In no way should these be given out in some method like BMAH, Trading Post, or whatever. Do the work required on another character in order to unlock the option of earning more.