Powers & Abilities - Who used haki and who not?

We know prior to the timeskip we did not see hardening or CoA at all. CoO at least was hinted at (breath of all things, skypea, all the sensing/feeling

What are the different hakis in One Piece? - Quora

Discussion - Haki Thread, Page 33

One Piece: The Strongest Busoshoku Haki Users, Ranked

Can conquerors haki improve when you are not fighting seriously with someone? - Quora

One Piece: What Exactly is Haki? Explained!

One Piece: 25 Strongest Haki Users in 2023 (Ranked)

Haki/Haoshoku Haki, One Piece Wiki

What is the next level of Conqueror Haki? - Quora

Can Luffy turn someone into rubber even if they have stronger Haki than them? - Quora