Geoson filter and function (element)

I have a hard time to implement this filter

// After this function call 'filtered' contains the filtered list of GeoJSON features
var filtered = geoJson.filter(
function(element) {
// We are only interested in elements which do not contain 'Mån'
return element.indexOf("Mån") != -1;
}
);

It will filter out all properties that does not contain the letter combination "Mån" (Monday). The result is that I want to see all polygons on the map that does not contain this letter combination. I just can't figure out how to use with this below... is it possible?

var no_monday = new L.layerGroup();
$.getJSON("..data/polygons.geojson", function(json) {
	
var vectorGrid =  L.vectorGrid.slicer(json, {
maxZoom: 20,
zIndex: 5,
rendererFactory: L.svg.tile,
vectorTileLayerStyles: {
sliced: function(properties, zoom){  //filter goes here!?

return {
weight: 0.5,
color: '#ffffff',
opacity: 1,
fill: true,
fillColor: '#ff0000',
stroke: true,
fillOpacity: 0.6
}
}},
interactive: true,
})

.on('click', function(e) {
var properties = e.layer.properties;
L.popup()
.setContent(
"<b>Comments</b>" + properties.T_1_txt + 
"<br>WeekDays: " + '<b>' + properties.Days_1 + '</b>' +
"<br>Date from: " + '<i>' + properties.Date + '</i>' )
.setLatLng(e.latlng)
.openOn(map);
})
vectorGrid.addTo(no_monday)
}) 

The issue mentions L.vectorGrid.slicer and the vectorTileLayerStyles.sliced callback, but names no repository file or test. Start by tracing how sliced GeoJSON properties reach that callback and check existing filtering examples or tests. Done means the requested property filter reliably controls which polygons are displayed.