Morning/afternoon all,
I've found some more information regarding this feature.
Teams uses Microsoft designer to generate custom announcement images for channel announcement posts. Microsoft originally published details of the feature in Microsoft 365 roadmap item 123501 and message center notification MC680348 (10 October). They then followed up with MC687791 (6 November 2023), which contained a significant detail in that a Teams Premium license is needed to use Microsoft Designer to generate custom images.
So, if you add this all together, it's only available in US(currently) and needs Teams Premium even when it becomes more widely available.
However I can still access it through classic Teams in a web browser, in my case Firefox ... as of Jan 5th 2024
