Thing is, projects that don't get touched for months and months are the least costly. Disk space is cheap; what's costly is compute time to process new commits,

Thing is, projects that don't get touched for months and months are the least costly. Disk space is cheap; what's costly is compute time to process new commits, new/updated/closed issues, new/reviewed/merged PRs, and so on. Inactive projects just sit there taking up disk space but basically zero compute time. So it would make no sense at all for them to delete old, inactive projects. (Which doesn't mean they won't do it: they might have hidden costs I'm unaware of, or they might make stupid decisions. People do make stupid decisions sometimes).