Currently, the only way to prevent entities from appearing in the App for Outlook Set Regarding lookup is to remove them from the Entity Definition of the app itself. This prevents use of that entity in other areas such as the Sitemap. We should have separate configuration of the Regarding lookup from that of entities that may be used in the sitemap.
Ideas Administrator

Following careful consideration by the product team, it has been decided that we will not be investing further in the old app. Instead, we highly recommend offering guidance to our valued customers on migrating to Copilot for an enhanced experience."