Here's the best answer I can give for those questions:
macOS - We want to implement this. But we have not done it yet because of other priorities
iOS - Apple makes it pretty complicated to set up split tunneling, so it's currently not on the roadmap at all. This might change in the future.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!