JOSM discards tags with these keys:
"created_by",
"converted_by",
"geobase:datasetName",
"geobase:uuid",
"KSJ2:ADS",
"KSJ2:ARE",
"KSJ2:AdminArea",
"KSJ2:COP_label",
"KSJ2:DFD",
"KSJ2:INT",
"KSJ2:INT_label",
"KSJ2:LOC",
"KSJ2:LPN",
"KSJ2:OPC",
"KSJ2:PubFacAdmin",
"KSJ2:RAC",
"KSJ2:RAC_label",
"KSJ2:RIC",
"KSJ2:RIN",
"KSJ2:WSC",
"KSJ2:coordinate",
"KSJ2:curve_id",
"KSJ2:curve_type",
"KSJ2:filename",
"KSJ2:lake_id",
"KSJ2:lat",
"KSJ2:long",
"KSJ2:river_id",
"odbl",
"odbl:note",
"osmarender:nameDirection",
"osmarender:renderName",
"osmarender:renderRef",
"osmarender:rendernames",
"SK53_bulk:load",
"sub_sea:type",
"tiger:source",
"tiger:separated",
"tiger:tlid",
"tiger:upload_uuid",
"yh:LINE_NAME",
"yh:LINE_NUM",
"yh:STRUCTURE",
"yh:TOTYUMONO",
"yh:TYPE",
"yh:WIDTH",
"yh:WIDTH_RANK"
This list in the source code AbstractPrimitive.java