Before Discussions, to my knowledge (I wasn't editing in this era), there were threads that allowed for wiki markup. When Discussions became a thing, the threads were converted. That's also why you'll see remnants like "Username wrote:".

"That's also why you'll see remnants like ..."

..or colored text, or blue links, or sometimes even html tags.

"Was it an old feature?"

Yes.

"can be enabled by contacting Fandom support?"

No.