Monzo-to-Monzo standing orders were mentioned as a future feature under “Recurring payments” in the “What’s next?” section of the “End of The Big List” blog post:
The feature also been suggested before here:
https://community.monzo.com/t/monzo-to-monzo-standing-order/32546?u=awjdean
Seems like a no-brainer to me - especially since it costs Monzo less money to send a P2P payment than via the Faster Payments service.