In this case I'll set aside my unwillingness to take action on any word classed as a plural.
Leavings is a plural noun without a corresponding singular word,
leaving. We don't say, "I'm throwing my leavings into the rubbish except for this leaving which can go into the compost bin." As you point out, Morbius, the argument is the same as for
earnings, which was
accepted back in 2010.
A number of apparently similar
-ings words have been raised in the past, but in line with my current approach of dealing with each suggestion as quickly as possible, I'll leave those aside for now rather than holding things up while I compile a list of all the words warranting similar consideration.
Leavings will be acceptable in future as a common word.