This is incorrect - it's an option to block block at the machine level, geared towards school computers and the like.
It seems to be a bugfix. There was already a way to block specific URLs on a machine, but it didn't correctly block the `view-source:` scheme.
Why would school computers want to block View Source? Shouldn't they be encouraging students to use the debugging tools?
A possible consideration is older quiz/testing sites that pass along enough hints in HTML source code/CSS styling that identify correct answers prior to actually sending responses back for evaluation.