People can still join to Friends+ instances by sending request to endpoint like:
(and you can easily use worlds endpoint to search for active and popular worlds, or just find it by name)
Then you can see list of instances including friends+ ones, so even without modifying a game you can just select random one and join to it using steam run options or just command line.
Also instanceid exposes owner user ID, so that can be used to find instance of selected user - like streamer one and troll him.
Also it can be used just to check where someone is even fi you don't have given person in friends list.
This should be quickly fixed...