On Tuesday, October 13, 2015 at 2:10:00 PM UTC-4, John MacFarlane wrote:

Best solution would be to ask these other processors to
support ... as closing delimiter -- since after all this
is a standard YAML. (Another advantage is that if you
use ---, many markdown syntax highlighters will highlight the
last line of your metadata as a title, which looks bad.)

I've asked over at the Jekyll github repo, where they clearly had this discussion and went for "simplicity" for users by eliminating the option of using dots, which had been added to the original (and now again current) practice of only allowing hyphens. Their issue 3138 has a rather derogatory reference to "bending to the will of pandoc", which completely ignores the issue of actual YAML standards, raised by several people.