I am somewhat in the same boat - my site isn't as old as yours (eight years old maybe) and I do put new content on it semi-regularly, but occasionally old pages

I am somewhat in the same boat - my site isn't as old as yours (eight years old maybe) and I do put new content on it semi-regularly, but occasionally old pages mysteriously become unindexed, and a handful of pages seem to be stuck in the discovered but not indexed bucket too. I periodically try and update them or expand on them a bit but usually it doesn't help get them indexed. And like you, my site isn't the most exciting content but it does get regular traffic.