So first: I know that it’s bad form to necro old threads in a forum, but on the other hand there’s not a lot of other ways to show my appreciation of the stuff people have written. Does anyone mind if I comment on old fics here?
also, I know a few people post their stories on AO3, are there any other sites people here post their fics on? I’m always hungry for more!
I think that an author will always appreciate a comment on their work, no matter how long has passed.
As for it being necromancy (I just wanted to type out the word), it could draw peoples' attention to something they haven't read yet?
While I'll agree it has historically been common courtesy (and in some places an actual rule) not to necro threads, we've never really cared much about it, here.
Go for it. 
Awesome, thanks both of you! I have a day off today so I’ll probably do some serious reading this afternoon :D