Allegro has the ALLEGRO_OPENGL_FORWARD_COMPATIBLE flag, which designates that the context must be forward compatible. However, there is a difference between a core context and one with the forward compatibility flag set.
OpenGL 3.1 removed most of the deprecated stuff from GL 3.0, but not all of it. There are still features marked deprecated in OpenGL 4.2 that are still usable. Wide lines, for example. A core 4.2 OpenGL context can still use wide lines. A 4.2 forward compatibility context cannot, because forward compatibility means "remove all deprecated stuff".
Because OpenGL makes a distinction between core and forward compatible, so should Allegro.