@doppelgreener Perhaps because of how close the Midnight Hunt release was to these new digital only mechanics, they do intend to update the CR for the digital effects and keywords, but do it this weekend as part of the MID update rather than do 2 CR updates a month apart? Though I would have said they should have just done the MID update early then, but they may have wanted to control mechanics spoilers that would be clear in the MID CR?

@Metagamer And if the CR does get updated for digital only rules, the on topic rules for the site might need to be revisited, because an argument could be made that since Magic the Gathering as a whole now includes rules that don't fit "Be playable by hand, by human players implementing all of the rules" that all of MTG is off topic (It would be a bad faith argument, but we've had a few of those on this site before)

@Andrew We already have the MID CR Update Bulletin and it doesn't have the stuff from Historic Horizons

@murgatroid99 Ahh unfortunate :( I really should stay more on top of the CR updates Gotten lax since I haven't been able to actually judge during pandemic.

"The Comp Rules aren’t being updated to explicitly support the Arena-only cards at this time." — Matt Tabak, 2nd August 2021

This second exchange is between April King and Matt Tabak on March 26th

So it looks like it was not contemplated, planned, or likely even desired to update CR to handle Arena-only mechanics.