As some have pointed out in their comments on the pull request, I'm not so sure it achieves much in terms of privacy. Although I'm not sure to what extent, wallets can be tell apart from each other when they're used to craft and/or sign transactions. Things like change output positioning, fee estimation, RBF signalling, etc., can be used to try to determine which wallet was used for a given on-chain transaction. Adding this "deniability" feature to Core only seems to extend its fingerprint, at least until another wallet implements it in a way that looks similar enough on-chain (which is not necessarily easy).