> knowing what is unused and what isn't

Or what I run into “Is this used …. and does it even ‘work’?!?!”

Sounds like Chesterton's fence¹.

[1] https://en.wikipedia.org/wiki/Wikipedia:Chesterton's_fence

At my previous job, one of the nicest bits of kit was the moribund code logging system. You put a function call into the moribund logging library from a bit of code you thought was unused. The first time a process hit that line of code, it would fire off a UDP packet to a server, indicating the process name and the line hit. (It's nice to still have test coverage on the otherwise unused code, so it's great to see which processes are actually hitting the code.)

Another system watched the version control system for who added that line of logging, and sent them status emails. After the moribund logging had been in production for two weeks, the emails turned into nag emails to either remove the code, or at least remove the moribund logging if you decided against removing the code.

Somehow along the way I think the color convention for showing diffs got mixed up. I think it should green for removals, red for additions :)