Since updating to Windows 11 the XBOX app has been unable to download or update games.

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today despite this Harrio

What error code are you receiving when you try to install games?
Also, right now which windows version are you using, 11 or 10?
Before deciding to reinstall windows, did you have the dame version?
And, how did you reinstalled windows? Via settings?

The issue is still present. The only change to the computer since being able to download my games has been windows 11 updates. I feel it is a windows 11 issue, but have no ideas as to why.

This does not answer the question, as updates are not detected by the Xbox App. Some games suffer greatly due to being unable to update (Forza Horizon 5, in my case, being completely unable to do anything the game should allow other than drive around). This is a massive issue, and I don't think it's a per-user issue.z

ALSO BRUH, THAT IS A WINDOWS 10 REPLY TO A WINDOWS 11 THREAD

In my case, the issue was caused by something I chose to do (and was advised to) with Windows 11 - throttling Windows Update download speeds.

Windows Update > Delivery Optimization > Advanced Settings

I had previously had issues with download speeds on my new Windows 11 PC and was told that Windows Update sometimes hogged all the resources. I went here and limited its bandwidth. Unfortunately, Microsoft neglects to tell you that it ties Windows Store and the Xbox app downloads into the same resources as Windows Update, so when I chose to limit Update I also kneecapped Store and Xbox. I reset everything there to what it was before, restarted my PC, checked to make sure my changes were saved, and then went to the Xbox app. Lo, it downloads my games normally now.

(Incidentally, this is another in a line of incidents I've had with Windows 11, Xbox app, or Microsoft stuff, where I went to the communities and support staff, no one could help me, and I went and found the solution myself)

It's not incorrect

Like the rest of Windows 8 apps, it has been discontinued since the beginning of the year and not updated to current protocols

It is attempting to communicate and failing.

The Xbox App been the one you're supposed to use since 2015 on Windows 10