In the past a centre diff with asymmetric torque split was in use. The example is from “few” years back in the 1986 Portugal rally which had both tarmac and gravel stages.
The Lancias had a front/rear torque split 25/75 on gravel and 30/70 on tarmac.
The torque split for the Audis was 40/60 all the time.
Malcom Wilson had with the MG Metro a torque split of 35/65 on tarmac and 45/55 on gravel. The other Metro drivers had 25/75 on tarmac and 35/65 on gravel.
The Fords had a torque of split 37/63 all the time.

The active front and rear diffs was banned from the beginning of 2006. If I recall correctly there was also talk about banning the active central diff but at least the drivers wanted to preserve it because it enabled the use of the handbrake. https://en.wikipedia.org/wiki/World_Rally_Car

The rule changes from season to season can be followed on http://juwra.com/seasons.html
The ban of the front and rear diffs is not on Juwra’s 2006 rule changes http://juwra.com/rules_2006.html
Jonkka must have missed it for some reason.

All the manufacturers attending the 1995 wrc season had an active centre diff by the end of the year.