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I finally tracked down where I had seen a "spot killer" using a dual section on/off switch. A 19" RCA from 1969. It may be there for customer satisfaction rather than protect the CRT.
Basically the cathode bias is removed and goes to near ground. I suppose if you have a set where it bothers you you could add a relay powered by the filament supply (as mentioned) or the switched power to the set.
I wouldn't get overly concerned with this issue. In a properly operating set it seldom is a problem. There would be a lot of CRTs around with a small area damaged in the center of the screen if it was really a problem and there isn't.
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