Looking at the code https://git.sr.ht/~sircmpwn/todo.sr.ht/tree/master/todosrht/search.py#L49 a simple way to implement this would be to use syntax like
label:!foo(ie put the negation as part of the label name), then check if the first character of
valueis the negation symbol and if it is, swap from
TicketLabel.label == labelto
TicketLabel.label != label.
Sometimes I'd like to see tickets that aren't labelled in a certain way (eg to see all non-bug tickets, something like
!label:Bug, or all tickets that haven't been reviewed:
!label:Reviewed-- the exact syntax doesn't matter).