It doesn't. We can't be even sure if /videoViewed still works, because they don't return a 404 if something doesn't exist. They just print "OK" every time.
Its really interesting how xfantazy and k2s are connected. There could be another way to watch k2s videos for free.
Basically, every video on xfantazy is hosted on keep2share. If you look at the api response for a video on xfantazy, it tells you the id that's hosted on k2s, along with other details, such as the video source.
The source file is always hosted on filestore.app and signed with the same parameters as on keep2share.
https://xfantazydotcom/graphql
Search query:
Video source query:
Alternatively, you can try to find the video you want to watch on xfantazy by searching for the same file name.