This is actually an intellectually challenging question.
Private companies can decide what to allow on their platforms so if they don’t want to allow something in their app store, I don’t see how or why they should be compelled to do so. If I have a grocery store, you can’t force me to stock a certain product.
Smartphones are a little different as has been pointed out. However, you do agree to the terms of service for the OS that operates on them. Arguably the OS is also a product provided by a private company, which can decide what to include or not include in them.
Theoretically, there’s nothing that would stop someone from developing their own smartphone and accompanying ecosystem for their own use or to offer for sale. Alternatively, an open source community could be created as well. How feasible is that in reality? Probably not very. Yet there could be quite a market for it with a significant disaffected user base that doesn’t agree with the decisions of the existing market leaders.
While highly inconvenient, you could also just have a generic flip phone for calls and texting and use a computer to access other content via the internet if you were to opt out of the existing smartphones.