Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1080 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 436.48
Broken: version: 2.81 (sub 13), branch: master, commit date: 2019-10-03 17:34, hash: rB6ac2a2061f8b
Short description of error
Unparenting a mesh that also contains an armature modifier removes the modifier
Exact steps for others to reproduce the error
The armature modifier will be gone after unparenting.
An unparent operator is NOT about modifiers. It's about hierarchy.
A rig is not necessarily solely defined by an armature modifier in a mesh pointing to an armature. There can be more complex relationships between different types of objects and modifiers. Clearing the armature modifier like it somehow makes it "unparented" assumes the most basic of relationships only and would be breaking rigs unpredictably since the user never asked for that explicitly. (What about mesh deform modifiers? Multiple armature modifiers? etc etc)