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.
Reading through the list of Google web search results, collinsdictionary.com said it was American English. I should have put my faith in chatgpt instead of google search.
