It seems that the tag chooser when creating a post simply displays “Search or create…” even if a user has too low a trust level to create tags.
The relevant code is: https://github.com/discourse/discourse/blob/master/app/assets/javascripts/select-kit/components/select-kit.js.es6#L300-L305
This attribute is set at: https://github.com/discourse/discourse/blob/master/app/assets/javascripts/select-kit/components/tag-chooser.js.es6#L16