In any kind of minimally-effective engineering team, engineers will only ever be working with and maintaining code that's "mission critical" in the sense you're describing here, there won't be any meaningful amount of "non-mission-critical" code worth considering or optimizing.

This is in some sense a didactic assertion: if it's not the case, then your engineering team isn't providing any value beyond what a bash script, gluing-together JIRA tickets and GitHub PRs via the LLM-du-Jour, could do autonomously.