So, currently to be an "active user" and be listed on Special:ActiveUsers, you need to have 1 action in 30 days.
I personally think that this would not fit the definition of "active", but that is up to each wiki administrator. Therefore, I think that a configuration variable should be (re)added, for example $wgActiveUserEditCount (which existed previously) to be able to allow each wiki to set their own threshold for an active user instead of being forced to stick to this default value.
I also propose we set the default value to something more decent, such as 10 actions (proposed by @Skizzerz in MediaWiki-General)