Apple

Lemmy supports it on your profile, you should see a checkbox for "Show NSFW Content" if you click your username in the top right and then settings (if you're using the default web interface, anyway). I believe this is disabled by default. Instances can also block NSFW for the entire instance, there's an "Enable NSFW" option in the admin settings. This is also disabled by default.

Memmy itself has two settings, which look like they're independent of your Lemmy profile: one which enables or disables NSFW entirely (which is disabled by default), and one which can blur NSFW if it's enabled.

I suspect they'll raise an eyebrow or two with the App Store reviewers, but they do have prior art they can point to as well as that section of the policy you just posted. We'll have to wait and see; I haven't checked myself to see if they passed review or not.

First submissions typically don't pass anyway from what I understand for a myriad of reasons, so don't be surprised if it does get rejected. They'll just update the app and try again.