That rule was settled 20 years ago by the French administration:
- there was maybe something like that scheduled for all/severall European members, but it was "buried" in the meantime?
- the reasons given by the administrations for their acts are like politician speaks: hardly (if ever) convincing


If I'm not mistaken, due to taxes and accounting regulations for French settled companies, it applies to all vehicles from all makes sold in France, as long as the seller's (seller's, not maker's...) head-office is in France.
Practically, if changes occur in July 2020 for a vehicle, that makes the concerned vehicle a "July 2020" one, for the French market, that's all. This is the same in most countries: all companies have to cope with the local rules, and the January 2020 Renault Trucks T is probably seen as a 2020½ model in Germany...

European normalisation? But German, UK and Swedish manufacturers to name 3 use a model year system so this would actually be a step away from harmonisation.

I find this whole thing somewhat unconvincing, I have to be honest because of holes in the argument like this, and really a French administrative ruling would only apply to French models anyway.

Not in France: in fact, one of the explanations given in 2000 was, besides the European normalization, the fact that most automobiles makes unveil the changes during all the year, the July change being nothing more than an administrative marker for years...
This said, French magazines give lots of "what's new in 2020 automotive lines" articles, from December to February, and so do web sites (see here or here).
And among the French makes, if PSA seems to sprinkle new models and changes all along the year, Renault apparently prefer to stick to January (or close), even for trucks.

But do they still apply the changes in summer? (as that was the main reason for using model years on the site)

As explained here, the "July to June" model year system was used until December 2000, in France. Since then, the calendar year is used as "model year".

Which countries are confirmed to use the model year? I first thought this was a North American-only thing, as per Wikipedia. But it seems that it is applied to various other countries on IMCDb and even local car dealers labeling cars as a certain MY if the respective vehicle was produced after a certain month (probably done for re(sale) value of the vehicle). What's official here and what isn't? It's becoming confusing.

Confirmed countries are: USA and CDN (according to Wikipedia, at least) and possibly Russia but I've seen model years applied to Japan cars?
Japan doesn't use this and it is unheard of, yet I still see the year used on Japanese vehicles produced in the latter half of the year.
Could we make an organized list of countries that use this system for future reference? Which countries are actually known for using it officially? That Wikipedia article seems North American focused and incomplete.