Well, we don’t know what they might have been, exactly, because we’ve never seen them, and now that Classic is scheduled, we probably never will.
And many suggestions have been made on the forums about what a pristine realm might be. But Blizzard themselves never mentioned expansion locks, AFAIK:
https://us.battle.net/forums/en/wow/topic/20743584206
We explored options for developing classic servers and none could be executed without great difficulty. If we could push a button and all of this would be created, we would. However, there are tremendous operational challenges to integrating classic servers, not to mention the ongoing support of multiple live versions for every aspect of WoW.
So what can we do to capture that nostalgia of when WoW first launched? Over the years we have talked about a “pristine realm”. In essence that would turn off all leveling acceleration including character transfers, heirloom gear, character boosts, Recruit-A-Friend bonuses, WoW Token, and access to cross realm zones, as well as group finder. We aren’t sure whether this version of a clean slate is something that would appeal to the community and it’s still an open topic of discussion.
https://player.twitch.tv/?video=v72784365
“The Pristine server idea was one that we floated as a general idea. It’s not aimed at the core legacy audience that wants the specific experience of old talent trees, the pace of the game, and the quest line that sends you across two continents, and all of that stuff. That’s not what pristine server would aimed at. It’s more a question of the social aspects of the game, the need for community, the speed at which you progress through the content.
“Those are things we are actually focusing on the regular game and I don’t know if that’s something that requires its own type of server to capture that. There’s some changes to level up experience we’ve already been making, I think there’s some we can continue to make in the future to improve the quality of that experience for a new player. Philosophically I think we want to make the experience one that lets you take your time and enjoy and have a well-paced experience if that’s what you want, but we can still allow people who want to rush through it the option to do that rather than have it be the default state for everybody.
“We know that dungeon finder, raid finder, all these tools make content super accessible, they also diminish some of the needs to form social ties to engage in cooperative gameplay and things like mythic dungeons and other pieces of content that aren’t necessarily hardcore in terms of challenge, but encourage the cooperation and community play are things that allow us to reintroduce that.
“It’s something we continue to talk about but obviously none of these ideas are dead or discarded. I wouldn’t expect to see anything on pristine servers in the very near future right now our focus is Legion.”
Pristine realms were intended as an easy, no-maintenance option for the part of the playerbase that wants a more social, less rushed experience. But they would have the current gameplay, and there was no talk of expansion locks.
As you rightly say, imbalance below cap would immediately destroy any concept of expansion locks anyhow.
Anyway, now they have committed to the Classic path, I think we can say that Pristine Realms have been removed from the shelf.