Warn that a Team has Guests when a member starts or replies to a conversation in a Team with Guests
A banner popping up at the conversation level reminding any member posting to Teams that the Team has Guests would be very useful to avoid members accidentally posting sensitive information to the wrong audience.
Outlook has this message reminding sender that the message being replied to includes external parties- something similar in Teams conversations would be great. Or like the banner that pops up in Teams when you include the whole channel or Team and are warned that you'll be notifying many people.
Having a naming convention and a label on the Team are not helping much when the member is jumping straight into a conversation, as the info flagging that the Team has Guests is not at same level (top of the window vs bottom of window).