Flavonals -- including catechins -- are the phytochemicals that give green tea much of its antioxidant potential. Caffeine, doesn't really have anything to do with the function of those antioxidants, but removal of the caffeine reduces some of the flavonals too.
According to one study tea published in 2003, the flavanol content of regular teas varied from 21.2 to 103.2 mg/g (milligrams per gram), while the flavanol content of the decaf green teas ranged from 4.6 to 39.0 mg/g. The antioxidant values varied from 728 to 1,686 trolox equivalents/g tea for regular teas and from 507 to 845 trolox equivalents/gram for decaffeinated teas.
So while there's a reduction, the antioxidant activity isn't totally lost.
Beyond that it's difficult to tell if decaffeinated green tea is more or less beneficial for humans -- some green tea studies use animals so it can't be assumed that the results are the same for humans.
Results of Studies Using Decaffeinated Green Tea
One study published in 2011 tested decaf green tea extracts (equal to about 6 to 8 cups of hot green tea per day) in overweight or obese men. The researchers found that when the participants took the supplements, they had an increase in the levels of epigallocatechin gallate (EGCG -- the best known of the green tea catechins) and lost some weight.
Another study published in 2010 looked at decaf tea and weight loss in women who had survived breast cancer. There was some weight loss during the 6 months, but not enough to be statistically significant. But, the women who took the green tea did have elevations in their HDL cholesterol (the good kind).
The most recent study on decaf green tea was published in 2014 and used a proprietary green tea extract product in the hopes that it would be beneficial for women who had persistent human papilloma virus and cervical cell changes that could progress to cancer. Unfortunately, the green tea didn't appear to offer any prevention at all.
So, decaf green tea may help a little with weight loss and it retains some antioxidants, so it's still a healthy beverage.
Brown AL, Lane J, Holyoak C, Nicol B, Mayes AE, Dadd T. "Health effects of green tea catechins in overweight and obese men: a randomised controlled cross-over trial." Br J Nutr. 2011 Dec;106(12):1880-9.
Garcia FA, Cornelison T, Nuño T, Greenspan DL, Byron JW, Hsu CH, Alberts DS, Chow HH. "Results of a phase II randomized, double-blind, placebo-controlled trial of Polyphenon E in women with persistent high-risk HPV infection and low-grade cervical intraepithelial neoplasia." Gynecol Oncol. 2014 Feb;132(2):377-82.
Henning SM, Fajardo-Lira C, Lee HW, Youssefian AA, Go VL, Heber D. "Catechin Content of 18 Teas and a Green Tea Extract Supplement Correlates With the Antioxidant Capacity." Nutr Cancer. 2003;45(2):226-35.
Stendell-Hollis NR, Thomson CA, Thompson PA, Bea JW, Cussler EC, Hakim IA. "Green tea improves metabolic biomarkers, not weight or body composition: a pilot study in overweight breast cancer survivors." J Hum Nutr Diet. 2010 Dec;23(6):590-600.