Do DC HUB Motors For Electric Bikes use regenerative braking?
For the most part, no.

The main problem is that you don't brake for very long compared to a normal battery charging cycle, so regenerative braking doesn't match the battery very well. Some systems use supercapacitors because they're designed for this sort of situation, but those are expensive and heavy in a bicycle application. Motors also typically don't work as well generating as motors, so they'll be less efficient, meaning a standard 250W motor might put out 200W as a generator.

The physics is pretty simple: a 100kg rider travelling at 36km/hr (10m/s) has E = 1/2 m v² = .5 x 100 x 10² = 5000 joules of energy. That's 5000 Watt-seconds. If you gently brake to a standstill in 5 seconds you're putting 1000W into your brakes, assuming no air resistance.

A typical battery is 36V and about 15AH, = 36 x 15 x 60 x 60 = 1,944,000 Joules or 2MJ, and takes 4-10 hours to charge. Pushing 5kJ into that isn't going to make a lot of difference - it's a 2000/5 = 4% recharge, assuming 100% efficiency. And you're not going to get 100%.

A much better scenario is either you pedal against the regenerative brakes, or you spend half an hour going down a long steep-ish hill, generating 200W all the way. Since a typical rider puts out 150-250 watts, for you to charge at that power level will mean pedalling very hard - you're going to need 300W or more to keep the bike moving, more like 500W taking into account all the losses. A world-class cyclist can put out 500W for an hour... and after that hour, the battery will be 30-50% charged.

This google search gave me a good explanation, a dismissal of the idea, some strongly held opinions and a couple of production bikes that have it.