I worked with Shopify support for weeks on this issue. I was told they added the captcha to my account page, but I never saw it during testing either. It wasn't on the account page or the second page. I have a third party theme too, so perhaps that was an issue. Support told me they hadn't seen the captcha work 100% of the time & advised I could hire a programmer to add one for me.
My fake signups would come in spurts, maybe a dozen one day, then nothing for a week. I finally had a programmer add an actual Google recaptcha to my account page & haven't had any fake signups yet, but it's only been a few days. I had several fake signups right before the captcha was installed. My programmer told me this issue probably would never been 100% fixed.
This is an accepted solution.
Based on positive feedback during tests we've enabled this behaviour by default.
Since this topic is pretty lengthy I am going to be marking this reply as the solution for awareness and blocking new replies.
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Thanks again all for your feedback on this issue!