The now tv app on the bt view box does not have the option of subtitles and the channel 4 app has not hd option either.
Both apps on phones and smart tvs have the options in place.
Why don't bt have them on the bt box?
The functionality of the apps isn't down to BT, it's the responsibility of the app owners. In this case, liability is purely down to Channel Four and NowTV.
I doubt it because the apps have all the features in other devices. So,now tv and channel 4 in purpose don't have all functionalities in the bt box?
Whether you doubt it or not, that's the situation. The apps are standard across all Youview boxes and aren't exclusive to BT TV so BT aren't responsible for their functionality.
This is why you have to research fully before committing to hardware, it isn’t as simple as “apps look and work the same across all devices” unfortunately.
I use an Apple TV 4K for a lot of my app based viewing. All of the core catch up players are in HD when watching broadcast content. Most catch up content is in HD apart from some stuff on itv HUB.
The NOW TV app for Apple TV doesn’t support 5.1 sound even if you have Boost. BBC iPlayer cannot offer UHD either as there isn’t any support for HLG HDR.
Sadly YouView seems to have quite basic versions of some catch up apps, the cynic in me would be inclined to think the app owners prefer to reserve the best experience for Freeview Play, which they all co-own.