I find it somewhat surprising that it’s worth the effort for Google to shut down the read-only version. Unless they fear some legal risks of leaving redirects to private links online.

Hard to say from the outside, but it’s possible the service relies on some outdated or insecure library, runtime, service, etc. they want to stop running. Although frankly it seems just as possible it’s a trivial expense and they’re cutting it because it’s still a net expense, goodwill and past promises be dammed.

Typically services like these are side projects of just a few Google employees, and when the last one leaves they are shut down.

yeah but nobody wants to put "spent two months migrating goo.gl url shortener to work with Sisyphus release manager and Dante 7 SRE monitoring" in their perf packet

that's a negative credit activity

Another possibility is that it's a distraction - whatever the marginal costs, there's a fixed cost to each system in terms of cognitive overhead, if not documentation, legal issues (which can change as laws and regulations change), etc. Removing distractions is basic management.