There used to be one in the diagnostics menu, which wasn’t advertised but was widely known.  Sonos closed it off because people who left their routers wide open to hackers caused a sensationalist click-bait style article claiming Sonos was vulnerable to hackers. 

Then put a damn power button on the top of the Sonos Beam where the other buttons are.  Doesnt that seem logical enough?

Personally, I almost never need to reboot my Sonos.  I long ago reserved IP adresses, so reboots aren't necessary, aside from an occasional HDMI handshake issue with my TV.  But that's only once or twice a year, hardly a reason to retool the manufacturing line to put another button on the Arc.  Have you tried using a cheap smart plug instead?

I would have to take down my 65 inch tv that is mounted above my fireplace and put the smart plug in there so no I haven’t done that.  I unfortunately need to have sonos support reboot my Beam about every 2 weeks which is when the tv loses connection to the Beam on average.  Maybe another solution could be to have a button that sends a request for sonos support to reboot the system for me then?  That way they still control the option and I dont have to waste 10 minutes explaining to each support rep to reboot my Beam every 2 weeks?

I personally would flip the breaker instead of calling Sonos.

I have no use for “smart plugs” much preferring a dumb switch that just works without external servers or power draw.

Tuck it out of sight, hang it from a screw or use a 3M Command Strip to stick it anywhere.

$13 or 2 for $20 and multiple colors.

https://www.amazon.com/gp/product/B07KFMNJLH/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1