Blender Version
Broken: git latest

Short description of error

The @= operator exists, but it does not change the lvalue. It needs to be fixed or removed.

Repro script:

m = Matrix()
mt = Matrix.Translation(Vector([0,0,1]))
m @= mt
m.to_translation() # Vector((0.0, 0.0, 0.0))
m = m @ mt
m.to_translation() # Vector((0.0, 0.0, 1.0))

m@= mt should be synonymous with m = m @ mt.