Tested with cloud-init-18.5-10.el8 on RHV, the issue is gone:
No error "KeyError: 'modules-init'" when run "# cloud-init modules --mode=final".
Move the status to VERIFIED.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2020:1719