How to reproduce:
1. Create a new project using the default 3D template
2. Import the Multiplayer HLAPI package
3. Observe the top-right side of the Editor

Expected result: "Preview Packages in Use" doesn't appear
Actual result: "Preview Packages in Use" appears when non-preview package version is installed

Reproducible with: 2020.1.6f1, 2020.2.0b3 (1.0.0, 1.0.6)
Couldn't test with: 2018.4.27f1, 2019.4.11f1 (Preview package warning doesn't exist)

Note: Errors are thrown when using a package lower than 1.0.6

The package has the dependency which is a preview package, nuget.mono-cecil. It would probably make more sense to the users if they could see why this package has this effect, clicking on "show me" in the "Preview Packages in Use" dropdown just shows an empty list.

(ps: unfortunately there is no non-preview version of this package)