If we add it to the standard library it will be for runtime uses (identify the call stack for which swapping in the new implementation or behavior breaks the test). It is pretty simple. You call 'godebug.New' in a global init to get a lookup keyed to a particular setting, and then you look at that setting at runtime. For example take a look at https://go.dev/src/crypto/x509/x509.go#L881 and look at the uses of x509sha1 a few lines later.

You just call x509sha1.Value() and make a decision about old or new behavior based on that. Bisect can then toggle that result based on the hash of the call stack to narrow down the exact path that leads to the critical decision.

You can ignore the IncNonDefault call a few lines later. That's about providing monitoring for whether any non-default (old) behaviors are being executed in a program, so that people can hook it up to something like prometheus and see whether they would be affected by changing to "always default behavior".