Do you think Android will one day become so restrictive that sideloading (installing apps outside of Playstore) will become extremely limited, or completely banned?

Um I still can. You use a file explorer app (I use an app called ZArchiver) and find the directory, tap on it and there is a pop up telling you to grant it permission, then you will need to locate it using a android system file explorer, and once you find it, you can grant permission to access the directory to the app. I know its hard to describe using words, but I can record my screen and show it to you.

Edit: In android 13 the data and obb diectory aren't actually restricted to third party file explorers, only the sub-directories, like the files used by an app (those directories starting with "com.") are resteicted and you need to manually grant permission using the method I said. I can access data and obb just fine using ZArchiver.

Solid Explorer has restored access to Android>obb and data folders since an update some time back fyi.

It is weird, I have a Redmi Note 9s and my phone comes with both a xiaomi files app and a google files app. I can't see the data or obb folders with the xiaomi one but I can read and modify them with the google app one.

So I assume this limitation is only for third party apps?