Reproduction steps:
1. Open the user's project

Expected result: there are no errors
Actual result: there is an error "error CS0121: The call is ambiguous between the following methods or properties: 'System.ObservableExtensions.Subscribe<T>(System.IObservable<T>, System.Action<T>)' and 'UnitySupport.Plugin.Extensions.Subscribe<T>(System.IObservable<T>, System.Action<T>)'"

Reproducible with: 2021.2.14f1, 2022.1.0b7, 2022.1.0b10, 2022.2.0a1, 2022.2.0a6
Not reproducible with: 2019.4.34f1, 2020.3.30f1, 2022.1.0b6

Workaround:
1. Go to Assets > Editor and press on UnitySupport.Editor.asmdef
2. In the Inspector window under Platforms, note that only the Editor is enabled
3. Enable any runtime platform (e.g. iOS) and press Apply
4. Note that error message goes away

Notes:
- Seems that System.Reactive.Core.dll is automatically referenced if the asmdef targets only the Editor