How can I remove mounted volumes from the list that are no longer attached?
So, I had to revise the title/question as the solution to my problem was not an answer to the question posted originally.

As far as I know, there is no way to manually edit the list of attached volumes manually. However, I managed to prevent mounted volumes to become "permanently attached" by borrowing parts of a solution to a similar problem.

The problem seems to be "Time Machine" related, I cannot say for sure though it seems like the problems I described was observed after mounting an old backup drive to access other files. I read a description of a similar problem and thought I'd give it a try and so far it seems to have solved my issue.


Remove the disk from Time Machine's list of backup destination
drives
Move/Rename or Remove the file "tmbootpicker.efi" from the drive's root
directory
Move/Rename or Remove the file /var/db/volinfo.database
Restart
Mount you backup drive and re-add to Time Machine for future backups


After a slower than usual restart (sweating bullets) I was able to mount drives again, tested a backup, mounted and unmounted drives and so far, they are ejected properly and removed from the output of diskutil list. I have not observed any other unwanted effects so far.

This solved my problem (even if this was not the initial question) and I thought I'd post it here if someone else experience similar issues.