Is it still against the rules? I remember it used to be, but I can't find anything about it. I suppose it counts as spam but it isn't under the spam rule,

In the Fun N Games forum and sometimes in the 'Scape Lounge, if a topic does not really require a lengthy response, it is not so much spam. If you constantly make 1-word posts in the 'Scape lounge especially in topics that require actual discussion, I believe action will be taken.

A one-word posts in forums in which post count will rise or in a thread that requires more than one word, will be taken as spam.

I am sure that if someone asks "Do you think my armor is alright?" and you post "Yes", it will be taken as spam. I was in support of no 1-word posting because seriously, there's so much more you can add to a post other than "Yes", "No", and "ok". Posting one sentence per post is not that hard.

The definition on spam in the rules is

Yes I know all about one word posts, you've told me everything I already know but the definition on spam in the rules doesn't say anything about one word posts.

Regardless if it's against the rules or not, it's probably just a good idea to not do one word posting outside of Fun&Games.