Not all changes in the final patch work all classic this is by design. For the exact same reason we don’t have access to all the raids, or guild banks or a ton of other examples like dailly quest hubs, they are phased.
Summoning into instances at the start of tbc was locked to the point of them stoping all summons in netherstorm due to the lore of the storm blocks summons there.
This is to prevent cheesing attunments by just eg summoning people to the Kara key shards or cheesing the flying requirements for the atunment.
Thou I haven’t tested if summoning is locked as a whole in netherstorm in classic instances summoning is correctly locked.
I understand why Netherstorm (or at least some parts of it) is/will be out of bounds.
I don’t understand why a warlock can’t summon into like… the 2nd dungeon of the expac. There is no “exploit” to bringing someone in Honor Hold to the halfway point of Blood Furnace.
Probably just a case of simplicity and workload. It’s a lot more work to create specific rules for every instance than it is to say yes/no to instance summoning.