No, like most other MMORPG games Ashes of Creation will not be released in Steam and it's been confirmed by directer of the game.

Steven Sharif

Steven Sharif, Director of the Ashes of Creation confirmed on multiple replies in game's official Discord that it's not going to be released on Steam.

[https://discord.com/channels/256164085366915072/257283936756039680/309089270096855041]

Update

Ashes of Creation will be on Steam.