Why do you need to make the dependence on Redis a part of the public API? You don't do that with the service API, so why do you have to do it with the library API?

If you're going to rewrite it in another language, what difference does it make if it was a library to begin with? There's no point in making all the upfront work in both the service and all the clients making them talk on the network on the off chance you need to do a rewrite in another language, when you need to rewrite the API layer too. The only thing that would over after the rewrite is the client code. Doing it risks having wasted time doing a network API level on both client and server for nothing, not doing it just risks having to do it when you need it. Doing a rewrite "without users noticing" has no real benefit here.