What does "standard" mean to you?

From what I can tell, that assertion is common in Agile advocacy.

But from my experience as a professional software developer in the 1990s, and from reading stories about software development, waterfall was not used at Apple, Be, Commodore, Data General, Infocom, Microsoft, VisiCorp, and more.

That's not to say no one used a (modified) waterfall model. Simply that I haven't found evidence that it's meaningful to say it was the standard development model. [1]

[1] Technically, DOD-STD-2167A's apparent bias towards waterfall was a "standard", but I don't think that's what most people mean in this context, and in any case it was replaced by MIL-STD-498 in 1994, well before the Agile Manifesto.

I put 'standard' in quotes and meant that it (and its derivatives) was widespread. I'm sure not all companies used it, perhaps it also varied by domain. My experience is that it was indeed common, and still somewhat is. I read at my first job 20 years ago we had to take a training on the V model, which is derived from the waterfall model. It has also always been common in my experience to write plenty of detailed specs before writing any code, for instance.

I don't see much value in debating this. It seems clear to me that the people who wrote the Agile manifesto have experience in working in such, or similar environment and reacted to change it. And clearly that way of working was widespread enough and enough of a pain point to strike a cord. Certainly, when I first read the manifesto it struck a cord and I understood exactly where they were coming from. I think it is spot on.

I can agree with "standard in [domain]", as there are many domains know nothing about.

What raises my hackles is the lack of any qualifiers, given how rarely it's used in the projects I know about.