Regular variadic arguments in general aren't used very often in C++ with exception of printf like functions. Not rare enough for majority of C++ programmers to not know about them, but definitely much more rare than their use in python. Main reason people know about it at all is printf. The "new" C compatible form has been supported since the first ISO standardized version of c++ if not longer. There haven't been a good reason to use the "old" form for a very long time. Which means that the amount of C++ code using deprecated form is very low.
Being deprecated means that most compilers and linters will likely add a warning/code fix suggestion. So any maintained project which was accidentally using C incompatible form will quickly fix it. No good reason not to.
As for the projects which for some reason are targeting ancient pre ISO standard c++ version they wouldn't have upgraded to newer standard anyway. So if new standard removed old form completely it wouldn't have helped with those projects.
So no you don't need to know the old form to read C++ code. And in the very unlikely case you encounter it, the way for accessing variadic arguments is the same for both forms through special va_list/va_arg calls. So if you only know the "new" form you should have a pretty good idea of whats going on there. You might lookup in references what's the deal with missing coma, but other than that it shouldn't be a major problem for reading code. This is hardly going to be the biggest obstacle when dealing with code bases that old.
Once they've obtained permission to do Step 1 they can add whatever they want, and in a few years for them it's time to repeat "Subset of a superset" again and get permission for Step 1 again. There is no Step 2, it's embarrassing that this keeps working.