Are questions about CRT TVs relevant in this SE community?
I certainly hope so because you better believe I have questions about CRTs!
They're relevant, and they're barely used any more. I don't see why reason why they shouldn't be.
Also, we've had a tag for them, crt-monitor, since almost the beginning of the site.
General questions about TVs are probably not on-topic. Questions like "How can I view digital broadcast TV on my old CRT TV" and "How do I connect this CRT TV to this old VCR" are really not related to retro-computing.
But questions about using TVs with retro computers would be. Questions like "How can I hook up old computer model XYZ to my old CRT TV" and "Why are the colors different when I hook up a CRT TV to my computer via RF versus Composite?" are clearly on-topic.
There is also a gray area: "How can I fix this broken CRT TV?" might be on-topic or it might not, depending on whether the OP intends to use this TV with a computer once it's fixed. If they don't explicitly say it's for a non-computing purpose I am inclined to give them the benefit of the doubt and call it on-topic.
No, they are not. At least not more than VCRs or Toilet plumbing.
After all, this is about Retro COMPUTING, not Retro Electronics or Radios from Yesteryear.
Having said that, it depends of course on the specific question asked.
Looking for some replacement diodes in a HF circuitry is for sure not, but looking for the reasoning why many home computers are tuned for channel 3 or 4 might be.
Or more general: A question about intrinsic workings of a TV set is not, while the same TV set in relation to a specific computer issue may qualify.