Is Ace Combat: Assault Horizon on Steam?

No. Ace Combat: Assault Horizon is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.

What platforms is Ace Combat: Assault Horizon on?

Ace Combat: Assault Horizon is available on PlayStation 3, PC (Microsoft Windows), Xbox 360.

When did Ace Combat: Assault Horizon release?

Ace Combat: Assault Horizon released on October 11, 2011.

Ace Combat: Assault Horizon

Oct 11, 2011ShooterSimulatorArcadePlayStation 3PC (Microsoft Windows)Xbox 360

Ace Combat: Assault Horizon — frequently asked questions

Is Ace Combat: Assault Horizon on Steam?: No. Ace Combat: Assault Horizon is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.
What platforms is Ace Combat: Assault Horizon on?: Ace Combat: Assault Horizon is available on PlayStation 3, PC (Microsoft Windows), Xbox 360.
When did Ace Combat: Assault Horizon release?: Ace Combat: Assault Horizon released on October 11, 2011.

About Ace Combat: Assault Horizon

Ace Combat: Assault Horizon is the 14th game released in the Ace Combat series. It took place in the real world, albeit in a different timeline from that of Joint Assault. Players took control of various members of Task Force 108, most notably William Bishop, as they shut down the threat posed by Andrei Markov and other rebels from Russia.

Summary via IGDB.

Ace Combat: Assault Horizon on Wikipedia

Ace Combat: Assault Horizon is a spin-off installment of the Ace Combat flight simulation video game series. It was developed by Project Aces and published by Bandai Namco Games for the PlayStation 3 and Xbox 360 platforms in October 2011. The game was later released on Microsoft Windows in January 2013 through Steam and Games for Windows – Live, with the latter notably being the final retail release for the platform shortly before its discontinuation. The title has also been removed from Steam listings.

Source: Wikipedia (text under CC BY-SA 4.0).