How to reproduce:
1. Create and open a new project
2. In the toolbar, navigate to Window > Package Management > Package Manager
3. In the Package Manager window, delete the “Timeline” package
4. Create and open a new C# script
5. Back in the Package Manager, install the “Timeline” package
6. In Visual Studio, enter “using UnityEngine.Timeline;”, at the top of the script
7. Observe the Error List in Visual Studio

Expected result: No errors are thrown
Actual result: CS0234 error is thrown

Reproducible with: 2.0.11 (6000.0.51f1, 6000.2.0b6), 2.0.23 (2022.3.63f1, 6000.0.51f1, 6000.1.8f1, 6000.2.0b6)
Not reproducible with: 2.0.9 (2022.1.0a1, 6000.0.51f1, 6000.2.0b6)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Notes:
- Full error: CS0234 “The type or namespace name 'Timeline' does not exist in the namespace 'UnityEngine' (are you missing an assembly reference?)”
- The error is only thrown in the IDE, no error is thrown in the Editor
- Restarting the IDE fixes the issue