I read that there is a bonus Mission: Springfield which can be easily activated by setting the Bonus Mission Value in the configuration document to {true}.

Is it me or does this look like some kind of prehistoric "day one DLC" with some "On-Disc" locked content?
I mean it's easily circumvented but still it feels a little odd...

It was on the 4th bonus CD only for pre-orders incl. a warfare tabletop game.
With the patch there you didn't need to manual change the bos.

Well Arcanum and Baldur's Gate (1 or 2, I forget) also had such bonuses. There are probably more.

Interesting to note that a decade or two later we have to pay for OnDisc™ DLC now.