Not sure if I’m following.
I used to add events by posting text + date in a topic. It will show in the [calendar] tag in the first post of that topic.
Then the setting I screenshotted let us show that calendar on the top of a category setting. That setting describes (as seen on the screenshot): display a calendar on top of a category. It gets the calendar from a topic ID you provide and places it above the category ID you provide.
Maybe something changed with the events plugin but I never used that.
All this is unrelated to events plugin I think, because I never installed that and it used to work as described above.
As shown here, events is related to forum wide events. Calendar is related to a topic based calendar, which is what we are talking about.