Unfamiliar prompt when inserting new prepaid SIM card into iPhone in China
I just have the sim card of China Unicom.
The text is "尊敬的客户，欢迎您使用中国联通业务！"
Which means "Dear customers, welcome to use China Unicom business!"

I don't think there will be any security issues.
It is a sim application that can handle business（e.g. order data plan) with cellular data off.
Go Setting-Moblie Data-SIM Application you can find more.

I don’t know more technical aspects.You can see the wikiSIM Application Toolkit