No, it was an integral part of the 'later' CRT's - I replaced plenty of older style CRT's, where the glass was a flat sheet simply in front of the CRT.
One of my workmates used to collect them as the sets were scrapped, and used them to make cold frames for his garden
Having said 'he' used to collect them, he was usually out on service calls, so I collected them for him, as I was the main workshop engineer.