Maybe a thread under this category for requesting, with a template that can be used? There should also be a demonstratable use for the category, and inactive ones should be archived.
As for the imagery-resources, it’s pretty active, mainly used for discussing imagery layers, and I’m sure there are people who’d be willing to moderate (I can help if needed).
As for users migrating from the mailing lists, I’m unsure. OSM has conventionally been more accepting than other FOSS related community on adopting new technologies (e.g. IRC is basically unused, Discord and Matrix are being used more and more), but the mailing lists have become so engraved into the community it might be hard for a switch.