This is me wanting to see when someone was last online to tell if they're active or to stop me worrying about them. Which... would be nice for me.

First time I've ever put something in suggestions- am I doing anything wrong?

Yeah support - if last active seems to be unpopular then I'm fine with forgoing it but still supportive of some sort of online indicator. It's so hard to tell if an account is abandoned or if they just don't post on forums.

(edited a typo)

Full support! Even if it's a little thing like "Last seen Month/Day/Year" if they haven't been active in more a week or so. "Last seen a day ago." or "Last seen 30 minutes ago." if they had been active in the current day or week. Something like that should be simple enough to add.

Support! What would also be useful is an online tab to the side of their profile or whatnot, to see who is online

That's a great idea.

For a first suggestion, you are doing great.

Support, provided that users are able to mark themselves as "offline" if they don't want their activity publicly visible!

Support only if it can be turned off or a user can set themselves as invisible. I know in general a lot of people don't like having their online status visible.

Support but more generalized like: online within the last week, within two weeks, last seen over a month ago ect.

Toggling it on and off is also a good idea

support w/allowing users to toggle it off if they choose

Only supporting if there's a toggle. I don't like my online activity showing and I am far from the only one.

Support if allowed to toggle - I would want to be able to choose not to show this.