So, the ability to edit a signature during message creation was changed in 16.x, I believe. Domain signatures are generally created by an organization, and pretty much set in stone. Many don't like users to edit their signatures if the domain administrator has created them and assigned them to users due to liability reasons, etc. So, the ability to edit them was limited, but not removed.
In order to edit a signature when creating a message in webmail, the signature needs to be unlocked. To do this, the toolbar in the message window should be expanded, then the user simply needs to unlock the signature by clicking the "Unlock Signature" icon. Here's a screenshot on where that is/what the icon looks like:
So, the option is still there if needed.