How do you tackle a backlog of deferred maintenance you didn't create? (opens in new tab)

I don't mean migration in the sense of switching databases or rewriting in another language. I mean that a codebase rots if nobody maintains it. Deprecations, warnings, upstream changes — each is cheap to handle the moment it appears, often a 10-minute fix (let's say 'in average'). But plenty of teams never catch them. No real CI/CD, no culture of reading changelogs or acting on warnings. So it piles up silently, and then at some point — usually when an infra or security team shows up with ti...