I haven't really seen this being used on any sites i've been visiting these days
Maybe you've seen them referred as quick links. If they are not used as often it's a pity for many reasons, one of them being UX, another being SEO.
Also what's the character limit for a steem post?
That's a good question.
You don't have to think necessarily really long. Think for example a post explains STEEM, SBD, Savings account, STEEM Power and Delegation, and the reader is only interested in one of them. Quick links help. Or if a post describes multiple features of an interface, and you already know most of them, but are curious about one or two you haven't used yet.
RE: Can I create a Table of Contents in Steemit.com?