That's one of the things that Blizzard does so bad and private servers try to solve, which is previous expansion content:

Let's say you loved playing Battle for Azeroth. Later Blizzard launches Shadowlands, the content for BfA gets irrelevant, the raids are not doable anymore at the same difficult, the power creep feeds in. Even if you buy the expansion just to get the “feel” on how it was, it's impossible.

MMOs like GW2 and even SWTOR does it way better, in GW2 content from Path of Fire is still relevant in the gameplay of the current expansion, while their PvE/PvP content is done by all players.

I feel Blizzard should just keep per expansion servers up and people can play “over and over again” the same expansion as much as they like.