https://github.com/tendermint/tendermint/blob/master/docs/app-dev/app-architecture.md (revision aaa060f) has a link "ABCI spec" that points at the nonexisting https://github.com/tendermint/tendermint/blob/master/docs/app-dev/abci-spec.md

Compare with the published page https://tendermint.com/docs/app-dev/app-architecture.html where "ABCI spec" points at https://tendermint.com/docs/app-dev/abci-spec.html which in turn says that that document is "DEPRECATED: Moved here"