> What’s the easiest possible way to cause stacktraces to also dump local variable information? I feel like this is a feature that should be built into the language…

That is built into Python!

    import sys
    
    def div(x, y):
        return x / y
    
    try:
        print(div(1, 0))
    except ZeroDivisionError as e:
        print("Div by zero!")
        print("Locals:")
        _, _, tb = sys.exc_info()
        print(tb.tb_next.tb_frame.f_locals)

This will output:

    Div by zero!
    Locals:
    {'x': 1, 'y': 0}

https://stackoverflow.com/a/5328139 has a more thorough implementation that will print you entire stack of local variables.