Do you think Android will one day become so restrictive that sideloading (installing apps outside of Playstore) will become extremely limited, or completely banned?
I am using an OnePlus 8T and can confirm that, even if the bootloader does not come unlocked by default, it is very easy to unlock and flash and can also very easily go back (backup is needed because locking back will wipe the phone).
Done myself.
I have a oneplus 8t also. It was very easy to unlock the bootloader and install lineage on it. There's also the MSM tool that unbricks the phone as well. There's also a way to update the firmware without installing OOS, which is nice.