The gtk3 docs give the following reason for the deprecation:

Menus are not meant to be torn around.

Yeah, they are meant to be implemented with web technologies and look like shit.

BTW, this tear-off style is probably quite old. Long ago, I used an early version of ANSYS (for Windows) which apparently was still close to its Unix original, and it had its menus pop up like real windows, with close buttons! They were nicely cascaded, but one could rearrange them.