twocats Yes, in the scheme of things, Flarum is quite new.
At its core, Flarum functionality is very basic, but then you can add on extensions for additional functionality. There are many extensions, which is nice, but at the same time, they can be of uneven quality because they might be developed independently... it's an open contribution model.
In this case, Subscriptions is a core extension but is missing this setting for some reason. The two options would be to find another extension that adds the missing functionality or I'd probably have to go deep into the code.
Flarum 2 may be coming in 2025, so maybe we'll be seeing some nice improvements then?