I mean I could see it, obviously moderators wouldn't use it. Idea is of course is bad actors will get banned or bored after a certain amount of time. Of course if it's not basing it on amount of posts or some level of activity, then that may just cause trolls to bulk produce accounts, and start using them as they hit a certain age.
When you do it by activity, then you run into the side problem. IE say someone is introduced to lemmy because there's a community focused on a specific topic they need help on. People create an account, post for help... but get no replies because obviously they haven't hit the activity threshold so, they effectively start out shadowbanned. They assume the lack of responses is because the communities aren't active and don't stick around long enough to figure it out.