Curious to me as backwards compatibility has been one of the strengths I hear Go proponents cheer.

Any idea what were the APIs that were likely to cause problems?

I don't think I've ever had that problem -- particularly once they introduced Go modules, which specified a specific version of a library dependency. My experience is like the author's: Even old random packages I wrote 5 years ago continue to Just Work when used by new code.

There are a handful of functions that they've deprecated, which will produce warnings from `go vet`; but that doesn't stop `go build` from producing a usable binary.

IIRC the code was pre-modules; it was a long time ago.