Don't talk about illegal things (abortion in certain places, DIY HRT, etc) on facebook messenger. Don't do it on whatsapp or via SMS or Instagram DMs or Twitter or Mastodon or Discord or whatever.

Use signal. Use a PIN on signal. Turn on auto-destruction of messages. If your data is not fully end-to-end encrypted, that data can and will be handed over to police and it will be used to prosecute you.