The composer toolbar has an audience button on X and Threads posts. Pick who is allowed to reply before it goes out.
Threads: everyone, accounts you follow, only accounts you mention, only your followers, or only the author of the post you are replying to.
X: everyone, accounts you follow, only accounts you mention, your subscribers, or verified accounts.
The button only appears on platforms that support this, so there is nothing to ignore elsewhere.
The last option on Threads only applies to replies, so it is hidden unless the post is one. If you set it and later remove the reply, we keep your choice rather than throwing it away, and the button shows the audience that will actually be sent.