The section that delves into the ‘effect’ is under the heading:
> The Montreal C++ Problem
And indicates that having old ‘stuff’ around when new ‘stuff’ arrives is the cause of ‘the Montreal Problem’:
> C++20 had a lot of good ideas, but lots of code predates that standard. And so drift occurs. You either don’t adopt the new way or you end up with a code base with more than one style. If you do the latter, you end up with the Montreal Problem.
(Emphasis mine)
The next section makes it seem like the ‘problem’ is about culture and language rather than, say, architecture:
> If you are doing work in the old-Montreal code section. It’s like a different dialect. You now need to know multiple dialects of the language and when and where to apply each one.
And then suggests that the new-Montreal old-Montreal problem is a divergence of community, which is ‘splitting’ people ‘apart’:
> So, how do you evolve a language without splitting it apart? This gets trickier with a whole community involved. ... And with that, the community fractures.
Overlaid with the idea that the solution is a ‘czar’ of some sort the metaphor feels clumsy and kind of inappropriate.
The title was changed, the one here on HN is wrong.
He doesn't say it's a problem for Montreal, so he doesn't need to explain that.
As a montrealer I think thee effect isn't really present here but whatever, if it helps him make a point.