The character clone service in WoW Classic was an interesting approach to how the game handled The Burning Crusade Classic vs. the vanilla experience; you could choose whether you wanted to advance to the next expansion or remain locked in the classic timeframe forever, and if you wanted to do both you could pay Blizzard money to have your character copied to both. However, that option is going away pretty soon, as the official site has announced the clone service will be permanently retired as of July 26th, 2022.
If you still have characters you want to have available on both server types, you can buy clones for 66% less money until it gets removed; after that, your characters will remain forever locked to your initial choice. If you haven’t made a choice for a given classic character, on the other hand, you will automatically be moved to The Burning Crusade Classic at that time. It’s worth remembering that there will be no remaining expansion-specific servers for The Burning Crusade when Wrath of the Lich King Classic goes live.