This is currently not working in iOS 15. See: https://developer.apple.com/forums/thread/691021
Sean Kent
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I can also confirm.
The magnifier (touch callout) is presented even when there is no text which can be selected, so this definitely seems to be a bug in safari.
The only thing that seems to prevent this is calling event.preventDefault on touchstart.
However, this isn't a viable solution since that also disabled scrolling, but it may be a workaround in some cases where scrolling isn't needed.