There is nothing missing. This is because "a great lunch followed by an afternoon at the pool" is not a sentence - it's just a long noun phrase. The words 'followed by an afternoon at the pool' are a reduced relative clause referring to the lunch. The full sentence would be something like this: "We enjoyed a great lunch followed by an afternoon at the pool". In this sentence, 'We' is the subject, 'enjoyed' is the verb and " a great lunch followed by an afternoon at the pool" is the object.

It is not a complete sentence. It's hard to say more without context. Native speakers often use sentence fragments when speaking or writing casually. This is common in blog postings and Facebook postings. In an English class, one would write: "I visited the Bernsteins yesterday. The summer weather was perfect. We had a great lunch followed by an afternoon at the pool." Informally, it would be very natural to write: "Visited the Bernsteins yesterday. Perfect summer weather! A great lunch followed by an afternoon at the pool." My answer to your question is that "is/was is missing because the writer is being informal... or careless." Another context in which sentence fragments are not only used, but are actually appropriate, is in "bullet lists" in Powerpoint presentations.

"A great lunch followed by an afternoon at the pool." If you used 'is' or 'was' before 'followed', the meaning changes. The original is advertising or promoting an event that has not happened yet. It is saying there will be a great lunch and that will be followed by an afternoon at the pool. For example, a school may give parents a brochure for an upcoming event for students that includes lunch and an afternoon at the pool. It is a promoted or advertised event that promises a great lunch and an afternoon at the pool. If you add "is", you are saying a great lunch is ALWAYS followed by an afternoon at the pool. It doesn't really make sense. If you add "was" you are saying about an event in the past. There was a great lunch and after that there was an afternoon at the pool.