As in, for any two Transform3D objects A and B i might encounter does Godot (4.1) always return A * B == B * A as true?
Alternatively is it approximately commutative, ie (A * B).is_equal_approx(B * A), in case there are situations where floating point imprecision messes the exact equality up.
No.
If you rotate 180 degrees about the x axis and then 180 degrees about the line y=x, you get a rotation anticlockwise by 90 degrees about the z axis.
If you do it the other way round, the rotation is the opposite direction (clockwise).
They only commute in quite specific circumstances.