Don’t think “killswitch” is the problem. Recommending platforms (for VPN use) that don’t enforce it are. The material that goes “hide traffic from ISPs” is. The public VPN providers that don’t implement “killswitch” are.
In short, if there’s a “killswitch”, the VPN client must implement it and optionally let the user enable it, regardless of the platform’s enforcement (or lack thereof).
In Android’s case, on a non-rooted device, the traffic “leaks” with “killswitch” is minimal, but without it, all bets are off. Recommending using VPN clients on Android, then, without a “killswitch” is diabolical and a dangerous suggestion.
In iOS’ case, from what I read on these forums, the “killswitch” only exempts Apple apps (discounting non-leaking but non-recoverable bugs such as loss of Internet connectivity when the VPN client implementing “killswitch” itself is “killed”), which is still okay in the grand scheme of Apple’s walled-garden.