Space Catitude 🚀It makes sense if you are responding to part of an existing conversation. It is possible that not everyone wants to follow the subject you are responding to.
So I said "always", because it's always "OK" to do that, though perhaps not always advisable.
As for the technical aspect ("content warning", "might affect threading"), that's all dependent on the technology platform, and you didn't specify.
It's rare that CW is really used as a subject line, TBH. And certainly not for threading.
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