No idea, but I've wondered. I've tried stopping before and didn't get benefits, but I was probably even worse back then. I drink quite a bit of tea (1-2 cups black tea, 1-2 cups green tea). I think there's a decent amount of histamine, but it's one of the few things left I enjoy.
How long did it take your brain fog to go away, and how severe is your ME/CFS symptoms in general?
Brain fog went away in a week. I was somewhere high 30% functionality, going out once a week for an hour or two. Yesterday I was out shopping and chatting with 2 of my grown kids for 7 hours, with no crash at all this morning.
BUT, there are layers to this. I stopped caffeine in February. I slowed way down on my tea intake, but increased my cocoa as it's turned colder, wanting a hot drink. My reactions to tea are much worse and faster than to coffee or cocoa, so it's likely that recovery from the tea took longer than coffee and cocoa. My last tea was a month ago, last cocoa/coffee a week ago.
BUT I also realized months ago that soy makes me brain dead. Since I drank soy milk daily for protein, this reaction was as hidden as the tea. I would not have noticed my success with tea/coffee/cocoa if I were still using soy.
ALSO in the past week I've added 2 things that might be important.
Garden of Life Sport protein powder is the only one I've found that agrees with me, it gives me 30 grams of protein, and it feels very nourishing.
Buhner's chronic fatigue herbal formula** feels restorative.
**Stephen Harrod Buhner "Herbal Antivirals" 2nd edition, page 110-111.