I found the English wiki page for Tag:access=agricultural misleading on the OpenStreetMap Wiki.
It displays the “Landwirtschaftlicher Verkehr frei” (except agricultural traffic) sign, with access=agricultural right above it. This gives the impression that the sign maps directly to access=agricultural. However, since it is a supplementary sign (Zusatzschild), that is never the case.
The examples using signs DE:250 “no vehicle” and DE:260 “no motor vehicle” are also poorly chosen. Here, vehicle=agricultural and motor_vehicle=agricultural are clearly the preferred options, not just “simple alternatives.”
First banning all access and then explicitly re-enabling every other mode of transport is error-prone—as can be seen in the example, where the allowance for horse-drawn carriages was forgotten for sign 260.
https://wiki.openstreetmap.org/w/index.php?title=Tag:access%3Dagricultural&oldid=2833027