Combination stainless steel chain/ galvanized steel anchor
For many years, bot on my own boats and those of customers in yacht service, I have removed stainless shackles as soon as possible and in place installed new Galvy items of the proper size.
I have temporarily used a stainless shackle to rig ground tackle for a delivery, and have never seen one fail. However . . .
It gives me the willies. Every once in a while I have put a wrench to a stainless fastener and had it crumble like a dry cookie. I don''t know why this happens or why the fastener right next to it hasn''t deteriorated, but it makes me eyeball a single stainless fitting on an anchor rode, be it a shackle or (shudder) a swivel, with suspicion. I see no gain in it.
I have never seen a good galvy shackle that was properly installed fail. Ground tackle is too important to experiment with.