You cannot really finish an entire campaign since most of them at the end are tied to raiding, and raiding starts at lvl30+.
-You can access most of the base game's maps (except Cataclysm continents like Hyjal, Twilight Highlands etc.).
-You can access TBC and do most of its continents like Hellfire Penansula, Zangarmarch and some of Shattrath rep grinding etc.
-You can access Wrath's 1st few zones like Howling Fjord etc. and few Dalaran quests, but Ice Crown has very little cuz lvl 20+ is required.
-You cannot access Cataclysm content as they are lvl25+, no Hyjal, Deepholme, Twilight Highlands etc.
-You can access Pandaria and do all of Jade Forest quests, and some quests in the zone next to it, cant recall the name. Not the rest tho, you can access Timeless Isles but wont do much since there are no quests and all mobs there are deadly.
-You can access WoD and do Garrison quests and mission tables and collect followers, finish Frostfire, Gorgrond and most Talador. But Nagraand and Tanaan are higher level so i dont advice going there.
-You can access Legion, get artifact weapons which are sadly useless as of now, do all of the main quests and storyline of Azsuna, Val'sharrah, Stormhiem and Highmountain. You cannot do Suramar as it needs higher level and you cannot do the Argus campaign cuz it needs 40+.
-You can access BFA and do the 1st few half of it, idk much about it since I did not bother much. You cannot use Heart of Azeroth nor Azerite related gear.
-You can access Shadowlands and do Bastion, dunno the rest of the other areas.
-You also do Dragonflight and port there via the Ship/Aircraft to the zone and do the 1st half of it and unlock adventure mode.
Goes without saying, anything TWW related is not playable, hell even Lorewalking event needs the expansion