I am going to be reserving a cloud server instance. If I am simply pulling data from a websocket and communicating with a normal REST api, does an instance type not supporting IPv6 have any implications? I'm looking at Alibaba Cloud and some of their instances are tagged with "not IPv6-supported".
I think that you should understand the role of IPv6 first because your applications normally will not depend on it. It's mostly related to the communications between the devices on the internet. Some people don't consider IPv6 during their implementation because it's also a new notion for them that they don't understand properly.
You can check this article on Alibaba blog to see more about IPv6
In simple terms, "not IPv6-supported" means, the instance will not be able to communicate over ipv6, which is an upgraded version of the current ipv4 protocol. There is still a long way to go till you would be communicating over ipv6 only. If you are looking to just use the instance for normal api use, it will not matter.
Unless you will be pulling any data from ipv6 only clients or someone using only ipv6 protocol will be pulling data from your instance, this will not matter. Most infrastructure uses dual stack, which means they would be both ipv4 and ipv6 compatible.
Google ipv4 DNS : 18.104.22.168
Google ipv6 DNS : 2001:4860:4860::8888
This is the difference between both protocols, so if you want to ask someone who will be using your API/instance to pull data, you may ask them by either mentioning ipv4 and ipv6 or sending these two types of IPs.