Regenerative braking is just one type of dynamic braking. If the energy generated from the traction motors during dynamic braking is dissipated through resistors ( DBR), it's called rheostatic braking ( generally seen in all of IRs conventional locos). If it is supplied back to the grid, its called regenerative braking. Both conventional locos and 3 phasers can have both rheostatic and regenerative braking.
3 phase locos also require braking resistors (DBR). This is because, even though it is said that they have "Energy saving Regenerative braking" feature, that fact is that AC power grids are very complex, and more than 50% of the time, even if the loco generates energy through dynamic braking, the power grid...
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will not be in a condition to accept it. So the generated power is not supplied back through the OHE, but it is dissipated through DBRs. In fact, the power savings with WAP7 that are being advertised now are only possible because our power grids are improving, without which this feature would be useless.
Initial imported WAM/WAG models of conventional locos also had regenerative braking, but this was omitted out during the development of WAM4 and later locos as the equipment required to efficiently convert DC output from the TM's to AV output for the OHE was still in development at the time and also our power grid structure was still evolving and there was no benefit of having regenerative braking on the locos at that time.