It's great to see some movement in this area! Thank you for your continued work on math.

How do I type an upright variable?

Does that mean math.var does not automatically make multi-letter variables upright by default?

What is the default font for math.op?
The body text font - this matches LaTeX.

I there a reason why this is the case, other than being LaTeX's default? To me, it seems that operators are "mathy" enough to be typeset in a math font.

Closes #5081 (no longer applicable)

Why is it no longer applicable? Wouldn't #5081 (comment) reproduce the issue?

I there a reason why this is the case, other than being LaTeX's default? To me, it seems that operators are "mathy" enough to be typeset in a math font.

Math fonts aren't designed to typeset runs of letters

The body text font - this matches LaTeX.

I'm not sure this is quite true. It does use a text font, but I believe the text font used for regular text and math operators can technically be different. Sometimes you want to use a font which more closely matches the actual math font.

6. What is the default font for `math.op`?
   
   The body text font - this matches LaTeX.

Not sure about 6. I think the text used for math operators should correspond more closely to \mathrm in LaTeX or upright in Typst, which is math font. Not the same as body text.

How do I type an upright variable?
$ upright(A) var(upright("blah")) $

I suppose upright(var("blah")) will also work? (It seems more logical to me and more consistent with #let lim = math.op(math.var(...)))

I there a reason why this is the case, other than being LaTeX's default? To me, it seems that operators are "mathy" enough to be typeset in a math font.

Math fonts aren't designed to typeset runs of letters

But what if I want to display an operator in bold or sans? Text font aren't designed to typeset mathematical letters. Multi-letter vars are typeset using a math font with this PR so using a math font to display runs of letters will still be common.

Is there a technical reason why math fonts can't typeset runs of letters? Or is it just that ligatures, kerning, and such are often only present in text fonts? If it's the latter, I suspect this would be mainly due to the precedent set by LaTeX and we should just ask math font authors to clone metrics and ligatures from the corresponding text font.

Not sure about 6. I think the text used for math operators should correspond more closely to \mathrm in LaTeX or upright in Typst, which is math font. Not the same as body text.

\mathrm is usually a text font, not math font. Here is a quick survey of common LaTeX defaults:

Therefore, using the body text font for math.op seems a reasonable default, and it is also consistent with how unicode-math treats \mathrm.

From this table (especially the last row) it seems that care is usually taken to have the \mathrm font consistent with the math font, irrespective of the main text font, as @Enivex suggested might be done.

So if the user changes the main font, but keeps the default math font (New Computer Modern Math), the best might be to keep New Computer Modern for math text such as operators.

Therefore, using the body text font for math.op seems a reasonable default, and it is also consistent with how unicode-math treats \mathrm.

I am unsure about this. Having a different font family for operators and variables seems very out of place to me. Even in the example you provided, \mathrm always uses a font from the same family, while with IEEEtran.cls, the main text font is a completely different font family.
At least in my quick testing with pdflatex, after loading a different text font, \mathrm matches the operator font but not necessarily the main text font. See this minimal example:

\documentclass{article}

\usepackage{amsmath}
\usepackage{palatino}

\begin{document}
\begin{align*}
	\cos(x) \\
	\mathrm{cos}(x) \\
	\text{cos}(x)
\end{align*}
\end{document}

Thus, with Typst's default settings, I would expect that operators and variables use NCM, while text inside math uses Libertinus.

Also, do the CM and LM Math fonts even contain the upright character variants? If not, then that explains why the roman fonts are used.

So it looks like we have (without manual override of the math.op font):

• The font for body text
• The font for math symbols and multi-letter variables
• The font for math operators and for text in math

while LaTeX has

• The font for body text and for text in math (with \text or \textrm)
• The font for math symbols
• The font for math multi-letter variables and operators (with \mathrm, typically used also by \DeclareMathOperator)

I think I find the LaTeX division more logical: sentences in math should use the body font, and var("speed") should default to upright and probably the same font as operators. This should be a text font that can be set separately from the body font (though like @MDLC01 I wonder if this could eventually be done with a math font...)

What font would/should numbers use? Should they match the operator font?

What font would/should numbers use? Should they match the operator font?

They should match the math/operator font to remain visually consistent. In my LaTeX example above, 1234 uses CM and \text{1234} uses Palatino.
In general, everything mathy should match while actual text should remain in its own font. That includes text that happens to intertwine with the math.