As an American, I've never seen that spelling, and I can't find much evidence that it's an accepted spelling. I'm curious how you came to that conclusion.
"Stymy" doesn't appear in Merriam-Webster [0] nor the OED [1]. Wiktionary [2] and Dictionary.com [3] do list it as "variant" or "alternative" spellings, but with no indication that it's an American spelling.
[0] https://www.merriam-webster.com/dictionary/stymy
[1] https://www.oed.com/search/dictionary/?q=stymy
[2] https://en.m.wiktionary.org/wiki/stymy
[3] https://www.dictionary.com/browse/stymy
