Ok yeah this was it. xrandr is getting all the expected modelines now
on kernel 5.8.0-49-generic. Thank you very much!
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https://bugs.launchpad.net/bugs/1924190
Title:
Display is
It appears you have a manually installed Nvidia driver, which is not
supported, and it stopped working around the time of the kernel update
from 5.8.0-48 to 5.8.0-49.
Please uninstall the NVIDIA driver you have and in future only install
NVIDIA drivers using the 'Additional Drivers' app so as to
