As to your broader point:
Technically, OSMUS Slack isn’t ephemeral, since that workspace is on a special plan that makes archives available and searchable indefinitely. However, it is easy for people to miss messages in a high-traffic channel like #tagging. Additionally, the workspace is behind a login wall, which makes it less suitable for keeping a record of community decisions.
I can’t fault @Local-Mapper for going where their proposal was likely to receive the most visibility among local mappers. The automated edits code of conduct states:
if your edit affects only one country or territory, then standard communication methods for the territory affected by the change
Common sense would include Slack among the set of standard communication channels. Over time, I’d like for this forum to supplant Slack for documenting decisions that have a lasting impact on the database. But we shouldn’t single out someone for following the rules as they are today.