This feature was announced earlier this year, but have taken some time to get implemented. This feature will give users a richer experience when chatting with users outside of the home tenant with federated functionality.
However, not all users are eligible for this feature. The official documentation has been updated with a description of how it is expected to work: Microsoft Docs
The short story is: Both users must be in a tenant set in "Teams Only" mode, and have these same settings themselves. You know this is the case when you can use features like @mentions, rich text and emojis in the chat.
According to the road-map, this feature should be well on the way to your tenants and land no later than "early December": 52394
However, not all users are eligible for this feature. The official documentation has been updated with a description of how it is expected to work: Microsoft Docs
The short story is: Both users must be in a tenant set in "Teams Only" mode, and have these same settings themselves. You know this is the case when you can use features like @mentions, rich text and emojis in the chat.
According to the road-map, this feature should be well on the way to your tenants and land no later than "early December": 52394