The realm has gotten much smaller and you no longer see server campaigns with the same sense of massive scale, or roleplay happening in the capital cities (sans Stormwind) during peak hours. There is plenty of activity going on in Khaz Algar, pertinent to the themes of the new expansion - Hallowfall is the zone that gets the most traffic. The Horde population is particularly small, but reports of its death are greatly exaggerated. After a while you find that you get to know everybody, and that goes a long way when you’re looking to get into something with longevity.
There’s a Discord server called Violet Vanguard where you can find lots of public roleplay events oriented toward action and research, established by a group of Dalaran crash survivors in-universe. I think that’d be a nice thematic taster for a new or returning player.
There’s also the Void-themed Assembly of Dusk, which might be conceptually appropriate if you’re looking to roleplay the character you’re posting on and want to seek out other dark magic users (you’d see me there):
I’d also suggest checking out Argent Archives, which is a bit of a server relic (it’s been going since 2007 and has changed very little in that time), but I can attest that lots of roleplayers still use it to make connections and share stories. We regularly get interest in our guild from people who say they found us via the AA.
The final arc of Shadowlands was set 36 years after the opening of the Dark Portal, and the beginning of Dragonflight was set in 40 ADP. You don’t need to worry about this too much - nothing particularly significant happened ‘off-screen,’ but it does mean that Anduin Wrynn has been missing from the public eye for a while, with Turalyon as the standing regent of the Alliance.
Hopefully that gives you a few useful pointers.