System Information
Operating system: Darwin-19.5.0-x86_64-i386-64bit 64 Bits
Graphics card: NVIDIA GeForce GT 750M OpenGL Engine NVIDIA Corporation 4.1 NVIDIA-14.0.32 355.11.11.10.10.143

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-01 22:29, hash: rBba188e721899
Worked: 2.83.5
Caused by rB0b49fdd0ee0d: Fix/cleanup Constraint poll function in liboverride cases.

Short description of error
once you add object constrain to library overrided objects you cant delate it,

Exact steps for others to reproduce the error
link an external objects to the scene.
in the Outliner select the object > ID data > add library override
add an object Constrain