I honestly had the same question years ago too, so I had check my "already read" list rlq and and the furthest I could find that still says "ongoing" ended in 2022 so it probs isn't a function anymore
you need to email an admin to have the status changed now, it's generally the uploaders job to tell a mod when a story is completed. (from my understanding at least, I don't think mods really have the time to go through thousands of listings on their own to figure out what's ongoing and what's not)
as someone who's uploaded stories before from start to finish, the above comment is the only way that ik of aswell. I've emailed an admin to ask for a status change on a story and it usually takes a few hours if not a day before the status is updated.
anyone have a clue how long it takes for mangago to mark something as completed? there's been a lot of stories i've seen that have long been completed but are still marked as on-going so i"m curious