I understand the philosophy behind that, but the user (and developer) experience seems subpar:
macOS and iPadOS (on a WiFi iPad): If you set includeAllNetworks you can ensure all the user's traffic goes through the VPN tunnel, with no loss of functionality.
iOS: If you set includeAllNetworks, you can ensure all the user's traffic goes through the VPN tunnel, but MMS will not work. Your choice is either "let MMS work, but don't guarantee the remaining traffic goes through the tunnel" or "kill user's MMS functionality".
MMS is currently treated differently by iOS when there is a WiFi connection, as seen in the logs - it diverts MMS traffic to the cellular network. Given how cell companies require MMS to be delivered, it doesn't seem out-of-line that it would also be treated differently by iOS if a VPN is active. And even if this wasn't the default, adding a flag to allow MMS to go outside a VPN seems really, really helpful here. (We already have an optional excludeLocalNetworks flag that only matters when includeAllNetworks is active. Perhaps an additional excludeMMS as well?)
While I can dream of a flag to allow MMS through, do you know of a way to allow MMS to go outside the VPN while otherwise keeping similar functionality to includeAllNetworks ("if... the tunnel is unavailable, the system drops all network traffic")?