Before 2.19, when omit is the value of a key, the key is removed. This only happens with dicts, no other data structures.
As of 2.19, omit, now not a special text placeholder, gets removed from all data structures, never showing up with the __omit* placeholder text.
As for:
I mean yes, you are feeding that to debug, and the removal in pre 2.19 is recursive. msg is a “module” parameter, and thus, the recursive removal will nuke the keys associated with the omit values.