Add a soft-deprecation mechanism for configuration options
I'd like to have a way to flag configuration options as "dubious but still supported", and warn users that those options might go away completely unless they explain to us that no, somebody is still using them.
There's a ticket about killing off old options that nobody is using (can't find it, though), and I've started nominating options for my own person sadness at https://docs.google.com/spreadsheets/d/1Np1vBm0jSvUqxwaQdhkqDmYWxe4E3RO0R5RoRMtpkQQ/edit?usp=sharing .
But unless we have a way to mark an option as "warn if set", then we won't have a nice way to do transitions here.