Just pull the fuse
My '88 has a working Shift Up light. It is a little annoying. It is a frightfully simple mechanism. It is a simple intake vacuum switch, so when vacuum gets too high the light comes on to tell you to shift to the next higher gear. There is a threshold value for engine braking that turns it off while decellerating (sometimes). There is also an electrical switch on the shifter to turn it off in 5th gear (since you can't shift up from 5th). When I drive with verve, it stays off. When I drive gently, it nags me, and shifting up doesn't really slow me down, so I'm experimenting with following its advice this week.
Why is it there at all? It is a simple hack to boost City economy test results for the EPA's CAFE mileage rules. The government is required to shift up when the car tells the driver to shift up when doing fuel economy testing. It added 10-15% to the City economy values used years ago, which is why Volkswagen, Saab, Honda, and a few others used upshift lights. Honda still uses one on the Insight. It's been so long since I last drove a new VW that I don't know if they still put their big amber arrow in the middle of the instruments anymore.
On my car the upshift light's fuse is clearly marked on the fuse box cover. I think it is #4 on mine. Pulling this fuse has no adverse impact on any other system, as far as I can tell.
Scott