new szMapName[64] get_mapname(szMapName, sizeof(szMapName))
if(equali(szMapName, "de_", 3)) return
Edit:
If you want to start plugins on specific map type:
1. Go to addons/amxmodx/configs/maps (if the folder haven't exists, create it)
2. create the file with the name like plugins-fy.ini (replace fy with any prefix of the map like de, cs,...)
3. Input any plugins you want to run on those specific map type.