I recently replied to a similar thread asking about Classic Era realms. Below is a Copy/Paste of that response:
Classic Era are the realms you’re looking for. They are dedicated to not progressing down any of retail’s past expansions. Many of the Classic Era realms have been ‘connected’ (blizzard’s version of merging) according to timezone region/server type.
Additionally, blizzard has opened free transfers to Mankrik in the PvE East Cluster and Whitemane in the PvP West Cluster.
Those realms (and the ones they are connected to) are in all likelyhood the most populated for a given faction/server type. I play Horde on the PvP East Cluster holding together Benediction/Faerlina/Heartseeker/Incendius/Netherwind.
For clarity, playing on (For PvP) Whitemane or the realms it is connected to (Anathema, Arcanite Reaper, Bigglesworth, Blaumeux, Fairbanks, Kurinnaxx, Rattlegore, Smolderweb, or Thunderfury) will have the same result since those 10 realms are effectively merged.
Similarly, playing on (For PvE) Mankrik or Ashkandi, Pagle, Westfall, or Windseeker will be identical since those 5 realms are connected.
One more thing: Solely for the purposes of Instanced PvP/Battlegrounds (Alterac Valley, Arathi Basin, Warsong Gulch) ALL of the NA realm clusters are combined/Cross-Realmed (Idk if OCE is included or not). So no matter where you play Classic Era, everyone will play together and have the same queues for BGs.
Classic Era communities usually sync their queues for BGs around the fridays/weekends. There is a specific discord for that sorta thing. I don’t have the link but perhaps someone else can provide it.
Good Luck and if you have any questions, ask away!