Wanted to not have some 60-something tabs open constantly and track artists and tags in a more streamlined way, only to discover the tag subscription feature I thought was there isn't there. Was it dropped or moved somewhere else? Or am I just being an idiot and there was never an option to favorite/subscribe to tags?

We've never had them on Gelbooru.

There's the My Tags list in your account options, but that's for when you're editing tags or uploading.

I've always thought that a feature like that would be useful... not necessary, but still useful...

I think Danbooru has that. Bookmarking tags/combinations and organizing them in folders is an alternative. It doesn't send a message when a new pic is uploaded, but it's better than always having all those tabs open. Of course, not everyone wants to use bookmarks and I would have less bookmarks if we had that.

They do, but it costs 20$ each year...

Er..no.
The upgrade for that is a flat $20 paid once, afaik. I have builder status, so I don't remember exactly. But I'm 100% sure you only pay once.