attributionPlainText
The attribution string as plain text.
RasterTileSpec.attributionHtml is HTML because that is the form MapLibre's attribution ornament renders. A renderer with no such ornament still has to show the credit — OpenStreetMap's and Esri's tile policies both require it — so rather than drop it or print © <a href=…>, the markup comes off here.
& is decoded last so an escaped entity does not turn into a live one.