Let It Be Written....
I don't think there is anything written in the FARs either, just in OPSPECs.
In the USAF, just about everyone says we are limited to 30 degrees. But when I challenge them to show me where it says that, there is no answer.
My best guess is that 30 degrees is the Air Force equivalent of SOP at an airline.
In the Air Force, the definition of 'aerobatics' is greater than 89 degrees of bank or pitch. If so doing, one must be in special-use airspace, and have a parachute.
The FARs contain a similar definition, which (I think) is 60 degrees of bank, +20, and -10 pitch. I think this is in Part 91.
Unless 121 contains a more restrictive definition, I think this would be it. The reality of most airliners being in the +2.5/-1.0 and +2.0/-0.0 range (clean and dirty, respectively) means bank would have to be limited for level-flight maneuvers.
I just can't recall ever seeing anything in writing that specified a maximum bank for Part 121.