Currently, no email groups (distribution lists) are supported for non-business plans, even with a custom domain where such lists wouldn't interfere with your wider namespaces. There are real, family oriented use cases for such lists that I think Proton should support, particularly because I've lost them when migrating from Google. The two examples I personally care about:

1. One small distribution list for my wife and me - we use this email address when creating accounts on web sites where emails might be time sensitive and important, such as banking. There are literally only two recipients.

2. For many years, each of our dogs has had their own email address on their tags, and each was a distribution list with the whole family as recipients. The idea is that if one of them ever gets out and we don't know, someone who finds them can email that address and there's a maximized chance of one of us seeing it quickly.

Whether you provided some specific set of restricted use cases (e.g. max 3 custom groups plus max 10 "everyone" groups), or simply recognized that with only six users supported under the family plan, there's very little opportunity for widespread abuse of the feature for commercial purposes and therefore just allowed 15 of them, it would be very useful for families to have a few of these email groups.