Statement directly from the church: “In recent publications for Church members, Church leaders have clarified that several substances are prohibited by the Word of Wisdom, including vaping or e-cigarettes, green tea, and coffee-based products.”

This statement does not specify green tea extract. However, it is heavily implied that green tea extract is also prohibited (though not explicitly stated) in the New Era article linked to by the church in the statement:

“Green tea and black tea are both made from the leaves of the exact same tea plant. The only difference is that the leaves in black tea are fermented and in green tea they’re not. They’re both tea and against the Word of Wisdom. Some drinks have tea in them but don’t advertise that fact, so always check the ingredients.”

I’m sure you could justify green tea extract being ok, but to me at least it certainly seems better to avoid it.