Irrelevant.
Someone new to the game isn't going to know who any of the races are, why are the subraces different?Someone who didn't play Legion wouldn't even know about the other four allied races being added in 7.3.5 (whether they are available at that point is irrelevant) because they wouldn't have played through the content that introduced them narratively.
It's 100% different. Invincible isn't a BfA piece of content like the new subraces are, it's a piece of WotLK content. So you run WotLK content to get it.In that case, how is locking them behind old reps any different than mounts gated behind old reputation grinds or that drop old raids? A new player will see someone riding Invincible, but will have to do old content to get it because they weren't here when it was relevant. Someone will see a player character that is a Lightforged Draenei and will have to do old content to unlock them, because they weren't here when it was relevant. There's no difference.
I don't care much either way but I do think it's weird that the game requires you to go back to the previous expansion and do outdated content in order to unlock content for the new/current expansion.