There is a research paper on this topic here: https://link.springer.com/article/10.1007/s11023-023-09644-y

A/B testing without consent is unethical. This doesn’t fly in any scientific fields, yet the technology industry doesn’t think twice about experimenting on users without consent.

I think it is mainly happening because companies don’t want to pay for user research/studies and would rather try and make assumptions about how their software is used based on aggregate data collection.

I use DuckDuckGo’s email hiding service. I have had sites reject signing up with a @duck.com email address. So services will definitely move to block Apple’s new subdomain only for aliased email addresses.