Grapevine is informal

Grapevine is informal, follows no set pattern of content.
What does “set pattern of content.” suggest?

“set pattern” means a fixed or predictable pattern.

“content” presumably refers to the content of the information transmitted on the grapevine.

This is not a complete sentence in standard English; it is either an error or a deliberate use of abbreviated English, such as in a note. The complete sentence would be “The grapevine is informal, and follows no set pattern of content.”