When an OP restricts a character to a certain tier, it typically means that they can't use abilities or techniques that would put them outside of said tier, unless otherwise specified. From my personal experience at least.
Technically, this would only be a High 6-C feat, anyway, since that's where Kurama is sitting at right now.
It's for the Perfect Susano'o only, I don't know who the hell changed it to "Susanoo" and removed the apostrophe. Actually, nvm, I know who it is lol. It should definitely be changed back, I'll ask some staff to do it later.
Anyway, the Perfect Susano'o is currently Low 6-B for fighting Sage Mode Hashirama's Wood Golem, but since the multiplier is likely getting axed, he won't be Low 6-B for long. It'll probably get downgraded to High 6-C+ via it's own AP calc.
This is why people shouldn't have opinions.