rsync command deletion error "IO error encountered -- skipping file deletion"

From the rsync man page:

--ignore-errors         delete even if there are I/O errors

That will address your problem.

Additional note about this issue:

vanished file should not trigger the condition IO error encountered -- skipping file deletion, but this often happens when you are creating a backup of a live system.

This is covered by rsync bug #7809 (https://bugzilla.samba.org/show_bug.cgi?id=7809) and should be solved in the upcoming release 3.1.0.

In the meantime use:

--ignore-errors         delete even if there are I/O errors

You should use an exclusion file to make rsync ignore the files that it can't read. The list can be generated with find . -exec test -r '{}' in the source directory and then massaged into the exclusion file format rsync wants.