The Apple TV app on third-party devices such as Samsung TVs has been updated to support HDR10+ streaming for both TV+ titles and iTunes movies.
Apple recently announced HDR10+ support in the Apple TV app, but then removed all references to HDR10+ again.
The announcement was premature but HDR10+ has now begun to roll out in the Apple TV app on Samsung Smart TVs, according to multiple users (1, 2, 3).
iTunes movies and TV+ titles
iTunes movie rentals and purchases such as Top Gun: Maverick as well as Apple TV+ titles such as Ted Lasso are available in HDR10+. This suggests that Apple is adding support for HDR10+ across the content library as a third HDR option in addition to base HDR10 and Dolby Vision. However, the film studio must provide to Apple an HDR10+ master file and only a handful of studios release content in HDR10+.

The Apple TV app now supports three HDR formats: HDR10, HDR10+ and Dolby Vision
Like Dolby Vision, HDR10+ uses dynamic metadata to improve the HDR picture on a scene-by-scene basis. You can see which TV models support HDR10+ in FlatpanelsHD's TV Database by using the HDR10+ filter.
Apple will next week launch its third-generation Apple TV 4K that also features HDR10+ support.