The concern is that people will assume the OSS is for both scenarios, so even if you close issues and have a PR bot rejecting PRs, you'll still get both support

The concern is that people will assume the OSS is for both scenarios, so even if you close issues and have a PR bot rejecting PRs, you'll still get both support tickets and (good|bad) PR from people trying to run the mainline system themselves. Supporting development on a made-for-self-hosting OSS project is the most user friendly option and allows them to avoid needing to handle the woes of public code.