How to reproduce:
1. Create and open a Unity Project
2. In the Window -> Package Manager install "Addressables" package
3. Enable preview packages (Edit -> Project Settings -> Package Manager -> Advanced Settings -> Enable Pre-release Packages)
4. Using the Package Manager install "Unity NetCode" package
5. Observe the console

Expected result: Console has no compile errors
Actual result: Console has compile errors

Reproducible with: 1.8.3, 1.11.2 (2019.4.15f1, 2020.1.14f1, 2020.2.0b12, 2021.1.0a6)
Not reproducible with: 1.7.5, 1.12.0 (2019.4.15f1, 2020.1.14f1, 2020.2.0b12, 2021.1.0a6)
Could not test with: 2018.4.29f1 (Unity NetCode not available)

Note:
-"Unity NetCode" package version doesn't seem to have an effect on the compile errors being thrown
-"Unity NetCode" package is hidden in 2021.1 stream's Package Manager. For testing, install "Unity NetCode" in an earlier version and upgrade the project