What should you NOT ask or talk about in a one on one?

I'm guessing you're talking about speaking to your manager. Generally there isn't much point talking about negative stuff if they've heard it before, it's unlikely to get actioned. I'd recommend being enthusiastic and positive and talking about what you want to do and volunteering for different tasks areas. As a manager you probably get sick of hearing negative moaning in IT even if it is justified and it'll be quite refreshing to hear someone with something positive for a change. This could lead to opportunities.

I don't think people should be so strict as to think of a general rule. There are always exceptions and sometimes there are managers that you feel like you can talk about anything with. The only things I could think of would be mental problems as I think many managers wouldn't have experience talking about these. A professional is much more suited for that. Even in this space, talking about a mental problem like burnout, might be acceptable if you have a *really* understanding and trusted manager.

The topic is you. Project details are fine. It's about what you haven't voiced out. Was there something about the project that you'd like to speak up on, or someone? Very often the unsaid things suddenly lead to things blowing up, and the one on ones are there to see and defuse that. Things like tech debt are also worrying and never spoken up on.

Sometimes 1-1s become idle chit chat and we forget important issues to bring up.

Chit chat is OK, even important for the relationship. BUT I hate thinking later on “oh I forgot to discuss X”. I think it’s good to track potential issues and talking points before 1-1s so you remember to address them.

I agree with you although if the meeting is booked and there is nothing else to discuss then it’s OK … but probably should say “this concludes the 1-1, I’ll write it up now and then we will move into talking about the technical details of X”

It's not the end of the world if you do, but if you do find yourself repeating the same news or information in several one on ones it's probably better to just discuss it in broader meetings.

Can you please elaborate what you mean by "one on one"? What context? (Work, Friends, Church, Pub, etc?)

Probably don't talk about your plans to leave the team or company.