I like the idea of a todo list that comes with a built in auto-delete.

You either do your to dos, or it auto-deletes them for you. No worry about it getting full, but also some pressure to actually get them done or they'll be wiped.

And if you're happy they're wiped, then you probably didn't need to do it at all.

I wonder if there's something like that already.

you can already do it, with Apple’s native Shortcuts app and Reminders app. You can set an automation to delete all reminders after a fixed time has passed with the ‘Remove Reminders’ action

Otherwise you can just use cronjobs with a small python snippet to parse and decide what reminders tagged with which labels to delete after X,Y,Z many days and hit the APIs of most major todo apps like todoist, asana, etc to just delete those tasks. Heck works with your local markdown based todo lists too.

This was a pretty nice feature for me in the now defunct arc browser. It simply deletes tabs that you don't interact with for 12 hours. If its important I'll open it again!