To reproduce:
1. Create a new project
2. Set Platform Support to iOS or Android
3. Close Unity
4. Open the project again

Expected result: after the project is opened, previously selected platform support is available
Actual result: platform support (Android or iOS) that was previously selected is not available anymore and states that it needs to be installed

Reproduces on:
2018.3, 2018.4, 2019.1, 2019.2, 2019.3

Doesn't reproduce on:
2017.4

Notes:
- Can't find an exact version when it started regressing as I am unable to reproduce the issue at all
- More info about the issue in the forum link:
https://forum.unity.com/threads/build-target-android-is-not-supported-switch-to-windows-or-exit-unity.625336/

Known workarounds:
- Using Unity Hub fixes the issue
- Before opening a project in Unity launcher, select "Learn" tab then back to project selection and open the project, the issue shouldn't appear
- Creating a new project and opening the wanted project through the top menu bar