I've also noticed that once you have an iphone if another iphone user adds you as a contact it stores your number as "iPhone" in the contact card type.
When I've switched back and forth from Android on a few occasions I've had to have users update that contact card for SMS to work properly. I don't think its necessarily an iMessage issue more iOS, but if anyone is still having issues with the switch after de-registering, check that.
OTA is explicitly marked as non-safe and is removed by Shadowsocks official. So Potatso will no longer support it and I would recommend you change your server.
It would be quite amusing if it couldn't work with AD, considering Zulip is based on Zephyr which is a very old chat protocol/application heavily dependent on Kerberos.
Just FYI, Zulip uses a similar UI model to Zephyr, but doesn't use the Zephyr protocol internally. (For example, Zephyr doesn't really work when one user is behind a NAT)
(I worked for Zulip, and have contributed to the open-source project)
When I've switched back and forth from Android on a few occasions I've had to have users update that contact card for SMS to work properly. I don't think its necessarily an iMessage issue more iOS, but if anyone is still having issues with the switch after de-registering, check that.