For <business reasons> we have in the order of 10^6 SNS topics in one account+region pair. In that account, while in that region, the SNS console is entirely frozen. I suspect they're doing some looped pagination polling in the background and fall over on our degenerate use case.
Edit: I can also back up the other parent comment - tagging still works fine (and I know for a fact the stability and scalability of the tag system is taken more seriously than most things within AWS)
There are likely a few factors at play here, including the fact that Netflix decided to go its own way in this regard ~15y ago.
Though I work at a smaller (but still large), newer company now we still also have our own stuff for this.