Should not happen apart from possible updates to the OS settings themselves.
Be safe with a backup …
I would highly recommend it. Anything can happen 
.
Be aware that TWRP just backups the state of the OS and leaves Androids “Internal Storage” out of it (if I understood correctly, the devs can’t guarantee to get it right 100% because Android made a mess of it).
So you need additional measures for your data … here’s what I do …
- I backup all my stuff in Internal Storage (pictures, contacts, ringtones etc.) locally with MyPhoneExplorer, because it works great and I additionally sync my contacts locally with it and it can do some more fancy stuff which might come in handy occasionally (e.g. screen mirroring or recovering contacts accidentally saved at Google to the phone) … but it is Windows only, I didn’t test it with Wine.
- I download all relevant update files to the phone (modem, LineageOS … OpenGApps for me).
- I boot into TWRP and backup the OS state before the update so I could go back if the update somehow turned out to be no good. I transfer the backup to my PC.
- I install the update files in TWRP, as I’m there anyway then.
- I reboot into the OS and go through my personal list of important features and check whether they still work.
I’m not sure at the moment about backing up Apps and settings … I didn’t bother until now whether I cover this as until now it only mattered if I changed the OS or installed from scratch, if in doubt I can set all that stuff up again if needed.
MyPhoneExplorer could extract Apps and install them again (one at a time, no batch), but I don’t use that feature.
I see Titanium Backup mentioned in that context pretty often in the forum.
I don’t know. I just install every update and thus don’t need to decide .