Short trace length yeah, that's good. But induced signal is induced signal... and the pins are huge and exposed.
You can reduce susceptibility here by dB if you use short traces and smaller pins and components.
By 10s of dB by pouring ground around it.
By 100s of dB if you put a shield can over it.
In exchange for no performance change in the oscillator? That's not even a discussion!
Sure -- I'll look at three motherboards I've got in my junk bin right now --
Dell Inspiron 5160 I believe -- Intel SuperIO chip, BGA top side, RTC crystal bottom side. Narrow 4-pin SO style plastic case. No shunt capacitors (probably enough internal on the pins). No evidence of removed internal planes. 8 layer board if I'm not mistaken.
Compaq 8510W workstation -- actually three tuning-fork (very narrow body, leadless(?) 4 pin) crystals I've spotted, one looks to be for the TPM (which, haha, Infineon SLB9635TT12 is a TPM controller from 2006 apparently!), has two flanking capacitors and a parallel resistor, solid ground underneath. One beside the Intel SuperIO chip, has capacitors and resistors nearby but unclear if they're connected. One beside the PCI bridge(?). Interestingly, this board doesn't seem to have a backup battery, though the others do.
Desktop motherboard, K7T Turbo2 -- VIA SuperIO, metal-can watch crystal, THT. Sort-of stapled to the board with a jumper wire. Seems they stopped short of actually soldering it, but the can is very likely grounded. Two connected capacitors, and a... 5.6M resistor?
So yeah, ground under all of them. Removed grounds would be just absurd in high density, high speed builds like these, and would have no effect on the poor crystal.
Tim