The employee can just turn off their phone today, or put it on do not disturb, or simply ignore the texts when not working. The boss could say "you need to reply to these texts or you will be fired", just like they could say "You need to take me off your $9.95 delivery list or you will be fired".
Also, is the point here that the employee will be happy to work outside of working hours if they are paid $9.95? If they get overtime they should be paid it based on the hourly overtime rate & policies for their role, not some strange one-time fee that their cellular provider collects for them.
The only problem I see, is that your boss is likely to use WhatsApp or else to find you, where such rules can't be enforced.
I could make an specific app go offline thanks to the NetGuard app (I'm not affiliated with it), it partially solves that problem. It must be manually turned on or off.
I personally think that's too much overreach as well. These groups tend to rot into culture echo chambers where you feel peer pressured to participate in a forced cheery way.