I know it's not a practice thought of highly on this DB but....

Mine is a 90 4matic also and I drove it in "test" (lever-up) for 2 years without incident...the only thing I noticed was the rear (limited slip) diff rumbled on tight, low speed turns (ie: like on 3-point turn, slow sharp turns), otherwise who knew? 99% of time the car is 2 wheeler anyways.

The test lever is not found on models after 90 or 91.

As far as reviving the 4wheel system after a long stint on 'test'...it might need to have a fault code reset, but it shouldnt. But if it does, disconnect a battery terminal 10 - 15 mins and it will erase the F/C; AND always keep an eye on the hydraulic oil resevoir (up forward on the drivers side, tall thin white plastic container with the word OEL on the black plastic dipper stick). If it needs topping up make sure you put the right stuff in it.

FYI it affects rear self-leveling system on TEwagons so....if yours is
a sedan no worries.