A change is coming to how Microsoft Teams orders channels and chats (amongst other things). You may be presented with the above prompt which will activate this change.
Rollback Options
Before we look at what this does, it is worth noting you can easily roll back using the following:
Go to Chat, the select the 3 dots > Customise View
In the View chats, teams and channels you can change between Combined or Separate
What is Different
The major difference here is the that the “Teams” and “Channel” Activity is located in the “Chat” area.
You can use the headings to filter the information.