[2021] Mage itemization is boring. Rabadons, Seraphs, Void Staff, Horizon Focus and Cosmic Drive all give pure stats only without noticably side effects when every other item has an effect in addition to the stats it gives

A few thoughts:

1. AP items are far from the only ones that only have a single output (dumb damage). You mentioned a few, but add Phantom Dancer and Rageblade to the list. What is Trinity Force but just a way to deal more damage? There are a lot of items whose primary output is "deal damage." This isn't unique to mages and it isn't a problem.
2. I don't think "more damage" is a bad output, even when there are multiple options that do it. IMO the point of the item shop is to present you with interesting decisions when you have gold to spend. Choosing among Horizon Focus, Deathcap, Void Staff, and Cosmic Drive as the best option is an interesting choice. No one item dominates the others in all game states. To me, that's the primary use of items. I think all the comments like "what do you mean, i love that the "item rework" gave me the same items ive been building for several years" are trolling. There are clearly decisions and if you don't think they exist, then chances are you're just bad at the game because you fail to identify changes in game states. Again, you're not only solving a math problem (boring, but can be ok) but a playstyle problem. Are you going to be assassinating Ashe or will you be forced to fight Sett in a sidelane? That's going to change your itemization and if you can't see that, that would be a you problem.
3. Some items do have play requirements, even with similar outputs. Horizon Focus asks you to lead with a stun or make sure you walk out of range. Seraph's asks you to have built a Tear early. Trinity Force and Divine Sunderer have optimizations around activating abilities late to maximize DPS and the list goes on.
4. I don't think items in a MOBA need to get too in-depth, especially when the primary gameplay is around unique champions. In some other possible game where all champions were blank slates and your unique character was more about the items it built, then I'd say yeah, League's items don't serve that game well. But in terms of asking you, the Syndra player, to choose the items in your build, it presents a solid spread of options.

Overall, I think League's item design is solid and I think mages have similar levels of richness in their items.

Yeah.

I can totally see the idea of "I want to build my champion in multiple ways" but there are already items for that. Like... just build a Spirit Visage if all you care about is surviving magic damage. Is the complaint that mage items have AP on them?

Saying that Seraph's, Void Staff, and Deathcap do the same thing is pretty silly. They do distinct things. They just also provide a lot of damage.