The "ability to disable replies" feature request on the Mastodon issue list is 5 (FIVE) years old: https://github.com/mastodon/mastodon/issues/8565
I just learned from @jdp23 that Bluesky, which has only existed for basically 53 seconds, will be getting this feature before us: https://techcrunch.com/2023/12/01/bluesky-rolls-out-automated-moderation-tools-plus-user-and-moderation-lists/
On the Mastodon Public Roadmap this feature isn't even in the "Planned" section: https://joinmastodon.org/roadmap 😐 (MAS-37 Restrict who can reply to a post)
If you too think this very basic safety feature, one of the most thumbsed-up on the Mastodon issue list, should be a higher priority, go to the Github issue and add your 👍 and a comment to bump it into the devs' awareness: https://github.com/mastodon/mastodon/issues/8565