Seriously? My previous phone, an Xperia Z5 Compact was the exact opposite. If you didn't back up the TA partition via a root exploit (which stored the unlock status and DRM keys which were wiped upon unlock) you'd be left with a camera even worse (no Sony proprietary enhancements) and lost Widevine forever (no Netflix for example). (Granted, there came out method which could restore camera via root and if you had a TA backup, Widevine keys too.)
That sucks.
I've only ever owned one Sony device, an Xperia SP, and I did not have a very good experience with ROMs on it.
There was this bloody line in the service menu that determined if you could unlock the bootloader or not (“Bootloader unlock allowed: YES/NO”). Apparently this changes from device to device, but if yours said NO, tough luck, you can't do shit about it.
Granted, there were custom ROMs that were somehow flashable on a locked bootloader (what kind of witchcraft is this?), but they were few and generally not very good. Last time I checked, the newest LBL (locked bootloader) ROM was 5.0 and it ran very badly. There are much newer versions for unlocked phones (even Oreo!), but since my phone was a “NO” one, I couldn't do much about it.
I believe the one that is currently flashed on it is a 4.4.4 CyanogenMod ROM. That was the newest LBL ROM that ran well.