What I took away from MMM was less concrete, actionable suggestions you can implement, and more "things to watch out for" in the development process. Things like the second system effect, and Brooks' Law (adding people to a late project makes it later) are red flags. You can't always avoid them (management will add warm bodies in desperation, you probably can't convince them otherwise). But at least when you see second system effect happening you can often try to mitigate it by design/architecture decisions.

I would also say the lessons around keeping teams small have been taken to heart in industry somewhat, e.g. in the notion of the two-pizza team at Amazon.

That said, some of Brooks' suggestions, such as the "surgical team" concept, are more products of their era and haven't aged well.