Bash has been changed to take /bin/sh under control of update-alternatives.

somewhat a noble goal, but while building live isos at leats, resulted in
errors, as there is an install cycle between bash (requiring
updtae-alternatives in the post script) and update-alternatives (requiring
/bin/sh to execute its post script, migrating data)

SO, unless u-a manages to run its post script as lua code without an external
dependency on /bin/sh, there is no chance to get this /bin/sh
update-alternative managed for the time being