I would like to know some idiomatic expressions synonym to "never-ending issue" as in such a sentence:
The deployment of IPv6 is one of the never-ending issues of the Internet
There are many synonyms the OP can use in the sentence such as everlasting/eternal/,unceasing issue.
The phrase "an ongoing issue" or an "endless issue" comes to mind.
"Persistent problem" or "persisting problem" sound acceptable.
Also, grammatically, I'd use plural in "is one of the never-ending issues ...".