Perhaps this definition:

> used to describe something that is enjoying great but probably short-lived popularity or publicity.

Though de rigueur could also work, meaning something that is required by the current fashion (or the then current fashion here).

Like the "fashion trend du jour." I still think using it as in this post is unnatural.

They almost certainly mean de jure, meaning “according to an official or legitimate proclamation,” as opposed to de facto.

That would make even less sense. De jure = by law (but by implication not in practice).

I don’t think it is used to mean literally by law. It means that the practice (being critical of other languages) was formally or explicitly encouraged by the community, rather than being an unspoken but common practice.

De jure is almost exclusively used when there's a contrast between theory and practice. "The UN seat is still held by the de jure government of Afghanistan, but de facto, the Taliban now have full control of the country."

Yes. The writer was clearly referring to the practices which were explicitly condoned by that organization. The organization was not literally a government, but the term still makes perfect sense when referring to the governance of an organization. The writer’s point was that being critical of other languages was not simply something that happened in practice, but was an explicitly condoned behavior.

No, they were saying the practice is demanded by custom, which is precisely what de rigueur means.

There is no organization of snobby Pythonistas etc mandating that its members make fun of PHP, and even if there was, de jure wouldn't make any sense. Quite the opposite, in fact, since it was de facto required to be critical of PHP.