There’s a lot of content between Cata and BFA, like a lot.
Legendary Cloak from Mists, Legendary Ring from Warlords, Mage Tower appearances from Legion, those three are the only major things I can think of off the top of my head here that are no longer available though
Which leaves massive amounts of stuff to do
For things like Mounts and Toys, check the Vanity Collection guides for some great pointers
Same for Transmog collections, there’s some great guides to check out that’ll break down where you can grab some nice looking kit.
For Legion Artifacts the only thing you can’t get anymore is the Challenge Appearance, the ones from the Mage Tower fight. The rest you can still collect.
Class Mounts you can indeed still do, you would need to do a bunch of Legion quests to catch up with the requirements (Class Hall and Broken Shore storylines) but it is doable if you put some time asides for it.
There’s certainly more out there as well, like I said there’s a massive amount of content on Pandaria, Draenor and the Broken Isles to collect and find, even a few secrets to discover