> PIC-based PCBs, no-one seems to actually be doing this
> isolating "ring"

I've never done it, and I wouldn't have the first clue how many
hundred PCBs I've designed and made that work just fine with
frequencies from 32kHz to 75MHz. The only concession I make
to the design guidelines is to always ground the crystal's case (to
the general Vss, which is usually next to the OSC pins)
--=20
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