Helium can leak into and between crystals in the metal inside the container and container itself. That process changes dimensions and mechanical properties of construction.

In this particular case I see it as a planned deprecation, sort of.

drives like this will typically be rated for a 5yr operational life.

If there is leakage, I wonder how long it would be before the data could not be recoverable? Could one transplant the platters and 're-charge' the Helium? Or just somehow recharge the old disk? Because unless / until we get some cheap lots-of-bit storage, all the 'stuff' we're generating will not be, in effect, archivable. Is this right?

Yay back to syringes and weird jigs and refilling our computing accessories with weird chemicals.

Dunno, but disks are consumable items - don't kid yourself otherwise.

I expect some drives to fail within 5 years, but I have many that still have data from much longer ago.

It would be a disconcertingly new phenomenon to find every drive in some old archival NAS failed simultaneously, or every one In a collection of external drives, because all drives last exactly 5 years with little variation, even when switched off, due to helium loss.

I believe helium is used due to it being more effective than air at removing heat. Vacuum is significantly less effective (as in, no convection in a vacuum)

I heard that it was because helium filled drives have less drag on platters (thus, are more efficient) than air filled drives.