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.
newsroom.churchofjesuschrist.orgThe Word of Wisdom is a law of health for the physical and spiritual benefit of God's children. It includes instruction about what foods are good for us and those substances to avoid. Over time, Church leaders have provided additional instruction on those things that are encouraged or forbidden by the Word of Wisdom, and have taught that substances that are destructive, habit-forming or addictive should be avoided. 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. They also have cautioned that substances such as marijuana and opioids should be used only for medicinal purposes as prescribed by a competent physician."
http://newsroom.churchofjesuschrist.org/article/statement-word-of-wisdom-august-2019[https://newsroom.churchofjesuschrist.org/article/statement-word-of-wisdom-august-2019]