You can sue against traffic signs here. The reason is that the legal logic presumes every road is either 50kph (inside town) or 100kph (outside town) - barring the autobahn, obviously. That's the max limit.

Anything lower than that is an infringement of your rights to drive your cat at 50 / 100kph. As such, you can sue the responsible government entity for putting up the sign.

As noted above, most speed signs and nearly all of them outside of town can only be put in place because a lot of people already crashed there. There's very little wiggle room in the law to put up a sign as a preventive measure, unless things already happened, and basically zilch of that for deer crossings, only exceptions would be things like declaring a 70kph limit infront of a school or nursing home.

Here's a fun one: There was a section of autobahn, standardly at unlimited speed, that had a max speed of 120kph put in, because people kept crashing there. A few years, just about a year ago from now, the signs were removed because there weren't sufficient crashes to justify having them anymore. Immediatly, crash numbers shot way the hell up.