Following along here, I wanted my new custom field to be searchable in maniphest and needed to add

"search": true

Documentation doesn't clarify that it is a boolean (I assumed it was based on the text, hopefully that's a valid assumption) but probably should and/or that it defaults to "false". It reads, currently:

search: Show this field on the application's search interface, allowing users to filter objects by the field value.

After enabling search I immediately hit T10315 as well (just for added note) - also hopefully tagging this as documentation is OK