How to reproduce (according to the user, not directly reproduced locally):
1. Create a new project (for ex. on 2020.3.0f1)
2. Upgrade this new project to a newer Unity version (for ex. to 2020.3.4f1)
3. Whilst the project is open, open another Unity project on the previous project's original Unity version (2020.3.0f1)
Expected result: Nothing out of the ordinary happens, the project opens without any issues
Actual result: The project on the older Unity version is filled multiple TMP errors such as: "Library\\PackageCache\\com.unity.textmeshpro@3.0.1\\Scripts\\Runtime
TMP_InputField.cs(3368,37): error CS0246: The type or namespace name 'CanvasUpdate' could not be found (are you missing a using directive or an assembly reference?)"
Reproducible with: 2020.3.0f1, 2020.3.4f1, 2021.1, 2021.2 (according to the user)
Could not test with: 2018.4.36f1, 2019.4.29f1, 2022.1.0a1 (lacking information)
Notes:
- After this occurs the older Unity Editor is now corrupted and can only be fixed either by a reinstall or by manually replacing the UGUI folder by a non-corrupted Editor alternative
- Does not reproduce when files are deleted from other packages such as 2d.sprite or 2d.tilemap
- Can be manually reproduced by deleting the metafiles in the com.unity.ugui folder found in the main Editor folder -> Data -> Resources -> PackageManager -> BultInPackages folder