A player sent us a deck that EDHcheck put in Bracket 4 for "an early 2-card infinite combo". Every combo in the deck was four cards. They were right to ask, and it led us to go through the whole bracket engine against what Wizards has actually published.
What changed
Only two-card combos count toward the bracket. Wizards' rules restrict two-card infinite combos. Loops of three or more cards are allowed in every bracket, so they no longer move a deck. They are still listed, and they still count toward power level.
Early or late now comes from Commander Spellbook's own tags. Spellbook tags every combo with how it fits the brackets. A combo it tags as fast and relevant is early and makes a deck Bracket 4. One it tags as slow, or as needing more support, is allowed in Bracket 3. Before, we read a mana figure that is 0 for most combos, which made combos like Exquisite Blood and Sanguine Bond (ten mana of cards) count as early.
Fast mana no longer pushes a deck above Bracket 2. Fast mana is not one of Wizards' bracket rules, and the strongest pieces (Ancient Tomb, Mana Vault, Chrome Mox and so on) are already Game Changers. Before this change, two fast mana cards made a deck Bracket 3, and three plus a Game Changer made it Bracket 4. That was our own rule, not Wizards'.
Mass land denial is now Bracket 4. Wizards doesn't allow it in Brackets 1 to 3, so a deck running it is Bracket 4.
Bracket 5 is now a cEDH profile. Wizards defines Bracket 5 as decks built for the cEDH metagame, which no decklist can prove. A Bracket 4 deck is placed there when it has an early two-card combo or a power level of 8.5 or more, and at least three cEDH signals: 5+ Game Changers, 3+ fast mana, 5+ tutors, an early two-card combo, a power level of 8.5 or more. The reason on the results page lists exactly which.
Bracket 1 and 2 are unchanged.
Power level does the rest
A deck can be Bracket 3 by the rules and still be very strong. That is what the power level is for, and it is no longer capped by the bracket for Brackets 2 and 3. When a deck's power is well above what its bracket usually scores, the results now say so. Wizards asks players to bracket up when a deck plays stronger than its list suggests, and that note gives you the words for it at the table.
What you might notice
Some decks will move down a bracket. They're mostly decks that were only Bracket 4 because of fast mana or a three-card combo, or only Bracket 3 because of two fast mana cards. If one of those decks was resting on the old Bracket 4 minimum power, its power level will now show its real number, which can be lower.
The full rules are on How We Score.