Almost nothing left
Decaffeinating black tea removes nearly all of its caffeine but not literally all of it. USDA puts brewed decaf black tea at 1 mg per 100 g, which comes to about 2 mg in a standard 8 fl oz cup, against 47 mg for regular black tea brewed the same way. That is roughly a 96% reduction, similar in scale to what decaf coffee achieves against regular coffee.
Close to caffeine-free, not quite
At 2 mg a cup, decaf black tea is close enough to zero that it won’t meaningfully affect most people’s sleep or a daily total. It is not, however, the same as a genuinely caffeine-free herbal tea like chamomile or peppermint, which USDA records at 0 mg, since those aren’t made from the tea plant at all.
Steep time still has some effect on decaf, the same way it does on regular tea, but the total amount of caffeine available to extract is so small to begin with that even a long steep won’t push a cup anywhere close to a regular cup’s 47 mg.
Where it fits in a day
A cup of decaf black tea is so far under the 200 mg single dose EFSA assessed as safe for healthy adults, or the 400 mg daily figure the FDA cites, that it barely registers. It matters most in pregnancy, where ACOG uses a 200 mg daily ceiling: switching regular tea for decaf is one of the simplest ways to cut a meaningful amount of caffeine from the day without giving up the drink entirely.