Hi there,
Although I'd really love to remove the MacOS version, as far as I know Itchio doesn't have an option to keep the MacOS build for existing customers and just remove it from new customers. And it doesn't sound fair to me that there's probably someone out there that bought the app to use it on an older Mac device, and I'd be removing their access to an app that works for them and that they paid for.
Having the option to remove it for future customers would make my life easier, but I think there's no better way to handle this than just leaving it there and processing refunds when needed and always being clear that the MacOS version is discontinued.