Should my "adb backup" include the system files or not if I'm planning to downgrade my Android version? I'm using Oneplus 3t on the latest Android 9 OS. I'm planning to downgrade it to Android 8 and hence backing up my data with adb first.

adb backup "-apk -obb -shared -all -nosystem -f d:/other backup/phoneadbbackup/backup2.ab


Now here's the problem. I could backup system apps as well. But I'm not sure whether or not that is a good idea. 

After I downgrade, there's a chance that the system apps would remain the same. If yes, then that's great because all the data could get restored back for the system apps. However, if there are a few system apps missing in 8 then could that error out the restore process? If the apps are same but the data was different in 8 then restoring data from 9 would cause errors, isn't it?

So my question is:


What does "-system" back up exactly?
If I'm planning to downgrade my OS then does the above command look good?
Nobody posted an answer over here so I'm going to post one. 

Backing up system files with ADB is a bad idea. The problem is that there can be small differences in system files between Android versions. And it's more likely to not work owing to the fact that it didn't work even without the system files (more details on that below). Based on the following experience I would recommend not backing up system files via ADB. In fact there was no use of going via ADB route in the end because ultimately I had to restore everything manually.

My experience with ADB backup: was not good. It created a file of 3GB which took upwards of 30 minutes. After restoring only 4 apps were actually restored but it occupied 3GB of my space. When I checked later via file manager then I noticed that it had created all kinds of junk files with missing apks or leftover fragments. Considering that you might have updated your os in your old Phone (multiple times), then ADB backup is going to copy paste all the file fragments for system files.

Hence, I don't recommend ADB backup/restore for system files.