It looks like this kinda/sorta/maybe improved in 3.5.0beta3 with FEATURE: allow edit custom flags - Pull Request #32344 - discourse/discourse - GitHub, but I’m not seeing it working — even on flags we’ve never used.
Prior to the custom flags feature, we had used site_texts to blithely change the meaning of some of the builtin flags 
… I do understand the rationale but this restriction seems quite paternalistic.