jQuery never really "supported" quirks mode, the unit tests were not ever run in quirks for example. A few of the offset and dimensional tests tried to account for quirks, but we gave up on that totally in 1.8 in favor of fully supporting the CSS box-model property.

Many Quirks pages could still use jQuery 1.8 successfully if they avoid asking for dimensional measurements, but I'm not sure why developers would still use quirks since it throws IE into prehistoric mode and has serious performance impacts.