Can no longer add People to a chat and include chat history in Microsoft Teams?

Having worked with Teams since it started and trained people on it all that time. . . what you are asking for is there and has always been there in a group chat. However, if you have a 1 on 1 chat - just you and one other person - it is just that, a single person chat. When you are in that chat and you add another person it becomes a group chat. To do that, it starts a new chat session with all 3 or more people. It does not include nor ask you to include the 1 on 1 chat history because that is considered private between 2 people. If you have a group chat with 3 or more people in it and you want to add someone. . . then you are always asked how much history in the chat you want to share. This is not new. This has always been the way that chat in Teams has worked.

What you are asking for would be an enhancement. You can provide that feedback from the Help menu in the lower left corner of the Teams client. One of the options there is Give feedback. You can provide your feedback directly to Microsoft from there.