When determining whether a creature could block all creatures the other is blocking, take into account evasion abilities (like flying), protection abilities, and other blocking restrictions, as well as abilities that allow a creature to block multiple creatures or block as though a certain condition were true. Take into account whether those creatures are tapped, but not whether they have costs to block (since those apply only as blockers are declared).
My impression would be that any static ability or modifier that passively changes rules on blocking would count, but activated abilities / spells that could allow a block if they were used, doesn't. Yet even that doesn't clear up war cadence because of the "block as though a certain condition were true". I think that the way it would come down is that war cadence requires you to actually pay the mana to remove the 'cannot block' passive modifier, and thus all creatures would be "can't block" up until you choose to pay that amount or not, at which point they would become either legal blockers or not, which might be relevant with extra-combat-phase effects like seize the day