It is true, don’t do it.
Even at huge companies like Google, lots of stuff was keyed on your email address. This was a huge problem so Google employees were not allowed to change their email for the longest time. Eventually they opened it up by request but they made it very clear that you would run into problems. So many systems and services would break. Over time I think most external services are pretty robust now, but lots of internal systems still use emails (or the username part of it) and have issues.
IIUC Google accounts now use a random number as the key. But there are still places where the email is in use, slowly being fixed at massive cost.
I don’t think that is true. Not much at Google really bought into the UUID hype. At least not for internal interfaces. But really there is no difference between a UUID v4 and a large random number. UUID just specifies a standard formatting.
