This leaves me wondering what the point is? What could it possibly cost to keep redirecting existing shortlinks that they consider unused/low activity already anyway?

(In addition to the higher activity ones parent link says they'll now continue to redirect.)

For a company also running a hosting service like GCP? nothing.

They already have plenty of unused compute /older hardware / CDN POPs, performant distributed data store and everything else possibly needed .

It would be cheaper than the free credits they giveaway just one startup to be on GCP.

I don’t think infra costs are a factor in a decision like this .

In another submission someone speculated the reason might be the unending churn of the Google tech stack that just makes low-maintenance stuff impossible.

To save face.