No. I've now read enough answers to conclude that there is no such command.
Easy answer:
Pressing 'Enter' while in insert will do it; but you're right, there oughtta be a key for it in command mode. I've wondered, too.
Since everyone has a favorite workaround, I will share mine. The assumption is that I will do anything to avoid having to reach for the Esc key.
ylprX ... where 'X' is the inserted character, which can even be a newline.
So, 'yl' is yank on char to the right, 'p' = paste the char, 'r' is replace that char; then you just type the new char. That's how much I hate using Escape.
You can split lines if you can create a regular expression for the location to add the split. For example if you want to split the lines at each semicolon, you can use the following substitution:
My solution was to remap the K key since I never use the default action (look up the word under cursor with "man"), and my previous editor used Alt+j and Alt+k to join and split lines, respectively.
:nnoremap K i<CR><Esc>
This rolls those three annoying keystrokes into one.
There's probably a more sophisticated way to also eliminate any trailing whitespace, but I prefer to strip all trailing whitespace on write.