I've hammered out a dent in a steel rim worse than yours and got it back to virtually unnoticeable. It took me about half a day (frequent stops to check I'd not made a mess of it) so it's not just a case of a few large whacks with a hammer, but it saved the rim and I'm still using one I did a few years back now.
If you've nothing to lose give it a go - but start with a light hammer! You'll need to loosen the nearby spokes before starting to hammer and tighten them up / true the wheel afterwards.
If the rim's ally then I'd seek expert advice. In my experience some will straighten, some will break, some need heat, some don't.
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