You can still use either a new OpenGL renderer or the old OpenGL renderer. This can be set with the GSK_RENDERER environment variable (e.g. GSK_RENDERER=gl)
I would assume it will also probably detect unsupported hardware and switch to OpenGL automatically but I don’t have any source to back this up.
I hope at least distros will make the switching automated because without it a lot of users will have issues, especially since Ubuntu and Fedora use GNOME by default.