If your personal files are stored in your /home directory, there is very little reason to believe that any of your personal files will be altered or lost. If you have personal files written all over the disk, then there are no guarantees.

Do note that upgrading the OS will also update a lot of the software and their individual settings files. There is a chance that things may temporarily appear weird if a config file is doing something non-standard. If you run a MySQL database locally, be ready for the engine to be upgraded to MySQL 8.0, which is a pretty big change from the 5.x versions that were standard with Ubuntu 18.04.

Generally I recommend people have backups in case of hardware failure and/or human error. When upgrading an OS, I recommend people have backups just in case things go sideways.