When sending message through UPS Console, SimplePush clients are not notified at all, because the console does not specify value for 'simple-push' property in the payload to /sender endpoint.

The /sender endpoint thus ignore the SimplePush variant.

I don't believe that's correct, because if someone specified a SimplePush variant in his application, he just wants those clients to be notified, thus potentially use default behavior, which is sending empty payload to SimplePushServer (be it our impl or Mozilla's one).