Lore-wise, it has been an extended amount of time since Legion, and the four races that we met are now an integral part of the Warcraft story. In terms of game mechanics, Legion will be two expansions old, and the pre-Shadowlands structure of the game is shifting drastically with (I'd guess) 9.0, thus it may have an impact on the old endgame structure, and the possibility of new players bypassing Legion entirely is a possibility now.
Therefore, I'd argue that the easiest thing to do would be to make the allied races introduced in Legion available to all players regardless of reputation or story completion. However, it would be wrong to do this without acknowledging the effort that the players who previously unlocked these races made.
So as a legacy reward, all players who fulfilled the requirements of any Legion allied race pre-9.0 get, for each unlock:
-A special color variant for the racial mount.
-A special color variant of the heritage armor.
-A payout of 3000g.
-A title.
In addition, getting the racial mount and heritage armor will still require fulfillment of the reputation and story requirements- they will not be immediately available to future allied race users.
I actually don't mind the rep grind, you know? The only one I couldn't see myself finishing is Highmountain and that's because the zone is so annoying to navigate- I think everyone who made the effort should be rewarded. But I also believe that you can't keep playable races locked behind outdated content forever, especially when things will be changing so much with the expansion.