I haven’t downvoted, but might be because third-party keyboards have been officially supported for many year in iOS. Your comment gives the impression they’re some kind of hack.

Thank you! No, that wasn't how I intend to mean it. If I recall correctly, the implementation was very restricted when the feature first launched in 2016. And if I recall correctly, it's not a feature which Apple would mention ever again.

Therefore, I have the impression that this is not a feature Apple would endorse much.

It's completely supported, you can get keyboard replacements on the app store.

It's just less secure to use them than the one built into the OS because the third party keyboards might run input through some web service instead of keeping it completely local.

There is a default-off toggle for whether to allow a keyboard to use the network, but of course it's so much more useful enabled that I expect everyone enables it.