I seem to remember a checkbox option (default was checked) for "Show Signature" when using a standard reply or creating a new post under "Additional Options".
Why was this option removed? It should be returned/restored. (This request is for the 2.1 branch)
I don't remember that option ever being built in, there is a mod for it though https://custom.simplemachines.org/index.php?mod=409
This was never an option in base SMF for it to have been removed.