Is the restricted entry only used for AO3 (and that one site using otwarchive)? I only found those two references so I'll say yes.
Another question: Feasible to add a 'subscribe' entry to ao3/otwarchive? This would obviously only work when logged-in. I
think it would be possible to get the status based on the button (when logged-in).
Code:
<li class="subscribe">
<form class="ajax-create-destroy" id="edit_subscription_12345" data-create-value="Subscribe" data-destroy-value="Unsubscribe" action="/users/Username/subscriptions/12345" accept-charset="UTF-8" method="post"><input type="hidden" name="_method" value="delete" autocomplete="off"/><input type="hidden" name="authenticity_token" value="removedauthtoken" autocomplete="off"/>
<input autocomplete="off" type="hidden" value="12345" name="subscription[subscribable_id]" id="subscription_subscribable_id"/>
<input autocomplete="off" type="hidden" value="Work" name="subscription[subscribable_type]" id="subscription_subscribable_type"/>
<input type="submit" name="commit" value="Subscribe"/>
</form>
</li>
<input type="submit" name="commit" value="Subscribe"/> changes to 'Unsubscribe'
But in general I'm not sure how useful this would be especially as it won't account for author/series subscribes.