Interestingly, while the Nintendo side of the pledge stands out for its odd fit and potential technical woes, the Valve commitment seems far more casual, with Gabe Newell telling Kotaku in a statement:

We’re happy that Microsoft wants to continue using Steam to reach customers with Call of Duty when their Activision acquisition closes. Microsoft has been on Steam for a long time and we take it as a signal that they are happy with gamers reception to that and the work we are doing. Our job is to keep building valuable features for not only Microsoft but all Steam customers and partners.

Microsoft offered and even sent us a draft agreement for a long-term Call of Duty commitment but it wasn’t necessary for us because a) we’re not believers in requiring any partner to have an agreement that locks them to shipping games on Steam into the distant future b) Phil and the games team at Microsoft have always followed through on what they told us they would do so we trust their intentions and c) we think Microsoft has all the motivation they need to be on the platforms and devices where Call of Duty customers want to be.

https://kotaku.com/microsoft-activision-call-of-duty-nintendo-switch-steam-1849862479