Why not remove this, as we've done with other --use-feature flags?

I kind of remember folks complaining that we remove feature flags that became always on because it was causing unnecessary churn and I was kind of agreeing. But yeah if we keep it we probably need some warning that it's going away one day. Not sure what's best.

Let's leave it in with a warning -- it's less churn for end users. However, I do feel like we should print these "ignored use-feature" warnings in a single central location, instead of scattering them across the commands. Do that seem reasonable?

If so, the only place that I can think of at the moment is Command._main, and... well, I trust your judgement on determining a good place. :)

@pradyunsg I have centralized the handling of always enabled features, so it will be easy to use for future similar cases.
I do that immediately after setting up the logger so we get the colored warning.