You're right that removing it without a word was poor, so let me correct that here.
Autopost worked for about four months and then stopped, twice, because two of the networks changed how posting works and both times I found out from a customer rather than from a changelog. Between the outages it was the most complained-about part of Fernpost, and it was never the reason anyone bought it.
So it is not coming back. What is there instead is a webhook on send, which anything like Zapier or Make can pick up, and that keeps the breakage on a service whose whole job is to keep up with these changes.
I should have written this in the 1.6 notes rather than letting you find it missing.