Setting a max downforce limit was a Max Mosley "Idea of the Week" a few years ago. His idea, as mentioned above, was to require a suspension with a combination of static ride height and stiffness so it would bottom out when a maximum load was added to it. This way it could be checked in tech inspection by simply setting weights on the car. On the track if the downforce exceeded this load then the car would bottom out and directly slow itself, or the car would scuff its underplank which would make it non-compliant in post race inspection.
Enter unintended consequences. One possible strategy is to put a speed limiter on the car to limit speed on the fastest straight. This would allow you to run a bigger wing without exceeding the max downforce limit. You would obviously lose time on the longest straights, but the bigger wing would make you faster through the corners. There is a tradeoff, but the obvious question is would this reduce overall laptime?
At the time I did a fairly complex spreadsheet simulator of laptimes for some typical modern F1 tracks, Turkey and China I think. The output was a chart of overall laptime vs speed limiter setting. The answer is yes: For the downforce limits that were being talked about (I think 1 or 2 times car weight) a typical F1 car on a typical F1 track would be quickest with a significant speed limiter. It wasn't a huge differece, I think the optimum speed limiter setting reduced laptime by 1 or 2 seconds compared to no speed limiter and a natural top speed. I think the range of optimum speed limiting was ~10 or 30 kph below the natural top speed. Anyway, obviously no team could afford to leave that last one or two seconds on the table so as a result all cars would end up running speed limiters on the longer straights, a pretty absurd spectacle for F1.
After a few months I never heard about the downforce limit again. I don't know if some technical person in F1 figured out the same thing I did and whispered advice in Max Mosley's ear or if the idea simply died for other reasons.
There is a way to make the idea work without speed limiters. It depends on monitoring downforce with sensors like WhiteBlue mentioned. Instead of a fixed downforce limit you would have a downforce limit that varied with the speed squared (since downforce is typically proportional to the speed squared). This would allow downforce to be limited, but with no incentive to limit top speed. It would have to be monitored with FIA approved sensors and telemetry, that sort of thing. Monitoring the sensors and preventing cheating would be difficult, but F1 has agreed on sensors and monitoring for KERS this year so maybe it's practical.
Thanks for reading. I'm proud that I conceived the speed limiter work around for the downforce limit. I think if the fixed downforce limit had been put into effect then the first season would have started out with one or two clever Ross Brawn type teams using speed limiters and all the other teams screaming bloody hell because it didn't follow the spirit of the rules (i.e they hadn't thought of it).