That API is public
I consume a lot of those API's from TfL, National Rail & Network Rail & it's quite normal for those feeds to contain data not for public consumption, even though the feeds are freely available.
Normally that information isn't to be presented to the public mainly to stop confusion - i.e. for NRE you can get a platform number for a train but it's suppressed so you shouldn't show it - mainly to stop people crowding on a platform before it's known the train will actually go there - e.g. late platform change. Busy stations can also suppress them again for crowd control.
The TfL one you can get from https://api.tfl.gov.uk/