Microsoft released an FAQ about their upcoming Flight Simulator which confirmed that the game will initially not be available on Steam or Epic Games Store.
PC players have been holding Microsoft in somewhat high regard recently as the company pushed the no-exclusivity narrative for the past few years, which sat well with gamers following the dust raised by Epic Games and their store expansion strategy.
However, Microsoft will not be initially releasing the Flight Simulator on Steam, making the game exclusive to their store and Game Pass. Technically, that's two ways to gain means of playing it but they are both tied closely to Microsoft.
On the flip side, this is indeed a first-party title so it's not exactly controversial at this point in time. First-party exclusives have been a thing for a long time, from Sony's games, over Valve's Half-Life to Epic Games' Fortnite and no one really lost sleep over those decisions. The controversy appeared when EGS started acquiring timed exclusivity rights from third-party developers which does not apply here.
Microsoft still left the story open-ended since they didn't explicitly state Flight Simulator will always be exclusive to their platforms. The FAQ itself states that the company's "goal is to have Microsoft Flight Simulator available on as many platforms as possible".
This means it is highly likely to appear on Steam and possibly EGS down the road, after the initial sales period concludes. Another bright side is that this will be an Xbox Play Anywhere title so players will not need to purchase Flight Simulator several times over - once copy will be enough for both PC and Xbox One.