Sometimes, you need to define the same filter for every selected layer when making a WMS request (e.g. when accessing user-specific data). In previous QGIS versions we had to define the filter using the format:
&FILTER=layer1:"column1" = 'value1';layer2:"column1" = 'value1';layer3:"column1" = 'value1'
With QGIS 3.8, we've simplified this, and now allow the filter format:
&FILTER=layer1,layer2,layer3:"column1" = 'value1'