Presumably anyone exploiting this would end up sending such messages to many iphones as well, where they could not delete themselves. If there are no suspicious messages arriving on iphones, that suggests it may not be being widely exploited.
they may also blast texts out to android only phone numbers (maybe if you gave your phone number to an android app)
Is that possible? I haven't touched iOS in a long time.
When you send a text message from one iOS device to another, it will be blue if it was sent via iMessage, green otherwise.
Again, not sure how this could be automated en masse, but I'm sure it's possible.