Phone, email, and in person are safe if you trust your counterparty?

Personal email, obviously. Never discuss anything of this nature on any communication medium that is owned by your employer. There's a reason "Direct Message" labeling overtook "Private Message" in chat apps.

Depending upon how sensitive I’d generally try to avoid written communications in general. It’s easy for even well-meaning friends, current or ex-workers to forward something without thinking it through.

Bulletin board systems?

On them. I absolutely would have no trouble finding / creating a space and the Twitter people are likely far more capable.