question

We have removed [thisentity] entirely, you can just use "*!*" now since entity-level permissions reference the entity itself implicitly

[SELF] always refers to the claimant now (so it IS valid as say /Members/[SELF]) but not as, say, a principal.

There's an unresolved issue in game manager where it doesn't show default policies of the group but if you add any policies there you will overwrite every policy on the group. For now your best bet is to at least get the full default policy set from the group by, in this case, trading your title secret key for an entity token with the GetEntityToken api, then using the GetProfile API to see the actual applied permissions to that group. Once you have those you can copy them into game manager (everything in the Permissions property).

And that specific policy in your question is already on the groups - it reads like this now:

{
 	"Resource": "pfrn:group--*!*/Members/[SELF]",
	 "Action": "RemoveMember",
	 "Effect": "Allow",
	 "Principal": {
	     "ChildOf": {
		 "EntityType": "title",
 		 "EntityId": "<myTitleId>"
          }
      },
        "Comment": "Allow entities to leave the group"
 }