Do you like Mastodon, but wish there was a privacy option to only post on your home instance without federating?

There's a fork of Mastodon for that called Hometown:

You can still post normal posts that federate, but Hometown also lets you post non-federating posts if you prefer.

It also has other differences such as rendering full articles from Write Freely (and other federated blogging services).

It's by @darius

p.s. Just to make clear, this information is intended for instance admins rather than end users.

If you're an end user who wants the option of instance-only posts, please contact your instance admin and send them the link to Hometown. why go half the way with a incomplete hack like this, when the Zot protocol (Zap & Hubzilla) can do detailed privacy settings per post in a federated manner. Hubzilla can also interact with non-privacy aware ActivityPub if you wish to do so. Not sure what you mean, but signing up on a public Hubzilla instance isn't any more difficult than any other website.
Disroot has some relatively recent how-tos on it:
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Mastodon is a microblogging site that federates with most instances on the Fediverse.