It's somewhat locked down as you need to apply with Microsoft for a Developer account, but you can technically sideload UWP apps onto your Xbox hardware by entering Developer Mode. https://learn.microsoft.com/en-us/windows/uwp/xbox-apps/devk...

It's not convenient and you need to reboot your console to go back to running commercial apps, but it is present. At least it's a one-time payment to get the developer account.