It's complicated, but doable: https://medium.com/blogging-guide/how-to-unclap-for-medium-articles-and-unhighlight-article-text-while-on-your-iphone-ios-eb8d0e376aac#:~:text=2.,is%20labeled%20%E2%80%9CUndo%20Claps.%E2%80%9D