With the release of Eldritch Moon, all sanctioned formats and the group that oversees the Commander format announced their updates to banned and restricted lists as well as any pertinent rules changes. This one may be even quicker than April’s update, as, well, it appears the spring and summer due to be relegated to the status quo. Which is fine. Thinking in all this heat is hard anyway.
The following changes are:
Standard, Modern, Legacy, Vintage, Online Formats
No changes, though apparently much was discussed. Oddly, though, it is the following rules posting that addresses Brisela as a Commander rather than here. (The short version on that is it works as you would expect, with only one of them being the Commander though the melded version still counting for commander damage purposes.)
Functional Errata and Relevant Rules Changes
For the most part, the rules updates are a bunch of additions and clarifications with the inclusion of Eldritch Moon mechanics, namely adding in some clarity around Emerge and especially Meld. It does also address and streamline a couple parts of casting a spell, mostly in relation to alternative costs having to look ahead to be cast in the first place. It has no impact on like 99.99% of games though and is mostly being cleaned up because rules.
Card-wise, the only thing close to a functional errata is that of Mimic Vat, which tweaked ever so slightly to function with what would happen if a melded card dies and the Vat triggers. Unlike how it works in all other cases, it will now take both melded card parts and you can choose to make one of your choice when activating it.
The explanations of the non-Commander Banned & Restricted changes can be found here.
The Commander announcement can be found here.
The full list of rules clarifications can be viewed here.