Hi superkookai,
I probably shared this text snippet with you, didn’t I?
Make a backup of your project folder by duplicating it. Save the duplicated folder somewhere else, maybe on an USB stick. To save some free space, you could delete the Library and Temp folders in the backup folder.
These two folders are save to delete.
If you want to delete other files, you have to ensure that you are able to retrieve the excluded content elsewhere. You could, for example, delete packages. The packages can usually be downloaded via the Package Manager.
However, the two important questions are: If you open the project in a year, will the packages still be available? And if they are not available anymore and if your project depends on a ‘missing’ package, will you be able to find a replacement?
The first question is not that important than the second one. If your project depends on packages, you want to ensure that they are included in your backup. The purpose of a backup is not to fill space on a harddrive but to make sure that you will be able to retrieve a working project if, for example, your computer breaks or if packages/assets get deleted ‘on the internet’.
Since each game is unique, there is no universal answer. Unless you do a lot of research and know what you are doing and how to fix things, I would not recommend to try to slim the backup down. In the worst case, you will get an broken backup.
If you duplicate the backup (without the Library folder) and open the duplicated project (not the backup!), Unity will recreate the Library folder. Then Unity launches the project with the empty default scene. If you are suddenly confrontent with an empty scene for no apparent reason, always check the scene name. In most cases, you just have to open your own scene to ‘retrieve’ your game objects.
See also: