> Some programs also have several variants of “without formatting”. Excel is a good example (https://support.microsoft.com/en-us/office/paste-options-8ea...). Which one should be the default?

Most of those are very specific versions of formatting or specialty features.

The only ones that would qualify as "paste without formatting" are "formulas" and "values" modes, and of those two I'd probably say "formulas" is a better default.