According to 9-to-5 Google, the standalone Street View app for Android and iOS will be discontinued as-of 21 March next-year:
I’ve always found it odd that on mobile it has continued to be a standalone-app for so-long; on the website version of Google Maps, “Street View” is simply a different mode. Making it a different mode within the Google Maps app makes-sense to me.
(Similarly, why does Google Earth still exist on Windows, macOS and Linux? They don’t even charge for the Pro version thesedays, and you can enter “GEPFREE” on older versions to unlock them. Why not just-merge whatever features are still unique in it into the Google Maps website?)