How to reproduce:

1. Open any (new) project
2. Select any gameobject
3. Open the Animation window (Window -> Animation -> Animation)
4. Click Create in the Animation window and save the animation
5. Enable Preview mode in the Animation window
6. Right-click on the Scale property in the Transform component
- Note this only brings up options related to properties in general and not any Animation related options
- Right-clicking Position or Rotation will bring up Animation related options
- Right-clicking on the Scale X, Y or Z properties specifically will also bring up Animation related options

Two other issues were also discovered with this feature…
1. The constrained proportions icon changes color as if I were animatable (red in rec & blue in preview mode), but it’s not. This visual feedback is used to inform the user on what properties are animatable, or not. This causes inconsistency and confusion for the user.
2. Copy/Paste does not work as expected in constrained mode. Hotkey seems to work, but Paste is not available in contextual menu (greyed out state). This is expected to work.

Reproducible with 2021.1 and above
Not reproducible with 2020.3 and below