Update:
The issue has now been fixed:
As of right now it appears that the customer tagging feature is back in business and working again on customer registration forms, I've just tested it using "name="customer[tags]"" and have been able to create a tagged account so all looks to be well.
Josh Wheeler
Shopify Support
I've been in touch with Shopify back and forth and this is the latest answer as of 10 minutes ago. It looks like they've now confirmed this as a bug and a fix is on the way:
Just wanted to reach out you let you know that the issue with tagging customers upon registration is one that we're looking into and should have fixed soon. While it isn't related to the multi-channel update itself, it does seem like the timing coincides with a change made here to be honest. Apologies for any problems this is causing!
I'll be in touch again as soon as we have a fix rolled out!
Regards,
Josh Wheeler
Shopify Support