While it hasn't been expressly resolved by the HTML working group to not propose new attributes with dashes, and while I don't speak for everyone in the HTML working group, I would assert that many members would object to a proposed attribute that included a dash (including myself).
There is also a proposal in the works to allow web developers to define custom attributes - much like custom elements - which would likely follow the same or similar rules around dashes, at which point I imagine the HTML spec would guarantee that no dashes would be used in "built in" attributes.
New attributes have been proposed that reasonably _could_ have had a dash, for example `popovertargetaction`, but instead they were compounded to one word precisely to cave out this path.
There is also a proposal in the works to allow web developers to define custom attributes - much like custom elements - which would likely follow the same or similar rules around dashes, at which point I imagine the HTML spec would guarantee that no dashes would be used in "built in" attributes.
New attributes have been proposed that reasonably _could_ have had a dash, for example `popovertargetaction`, but instead they were compounded to one word precisely to cave out this path.