More to the point--and no, I will never stop harping on this, because we have GOT to stop leaving our strongest points in the drawer--it doesn't matter if you heard of this convention before joining AO3 or not, because it's in AO3's tagging FAQ.
[id: the "How do I tag a romantic or platonic relationship?" section of the tagging FAQ here.]
"But Jo," you may argue, because you're wrong. "There's no way to find that without digging through site FAQ menus, and that's really inaccessible!"
that when you go to post a new fic, and you go to put in those relationship tags, you see this
[id: the Relationships field]
and that tooltip, the one THERE TO EXPLAIN HOW THE FIELD WORKS, links to the Relationships segment of the tag FAQ, which explicitly lays this shit out.
I don't care if you don't know fandom history. I don't care if you've never heard a goddamn word about the spirk shippers. I don't care if you've never been exposed to fandom culture in your life. It is, frankly, not fair to expect those things of everyone.
What is entirely fair to expect is that you will READ THE INSTRUCTIONS PRINTED NEXT TO THE FUCKING BOX, actually. Forget fandom conventions. It genuinely doesn't matter whether you agree with or respect fandom conventions. This is a site policy. This is explicitly how tagging on AO3, specifically, works.