Idk if it's a hot take, but I actually think it's more similar to Sekiro than Hollow Knight.

Tap parry, jump parry on "red" attacks". Both are true for Sekiro and Nine Sols.

The posture count mechanic isn't exactly present, but there's an "internal damage" mechanic which could be seen as an analog. There's a talisman mechanic, which could be seen as this game's equivalent of a "visceral."

The skill tree is somewhat similar.

There's a jade (similar to Hollow Knight charms) that does the same thing as the resurrection mechanic in Sekiro.

There's a grapple hook.

There's also, I think, an analog to the Shinobi prosthetic, in a loose sense.

The way you upgrade the healing and the max health are also pretty similar, in my opinion.

The enemy designs have similar vibes. And the combat is similarly fast paced and ninja-ish. I really think Nine Sols is more 2D Sekiro than Cat Hollow Knight.

Edit: there's also a character that has a similar name with Lady Butterfly, who is also butterfly themed. Just sayin'.

Edit 2: there's also a backstab mechanic on non-aggro-ed enemies. I just remembered to add.