You can also just buy an old Airport Express base station and repurpose it for AirPlay. The old 802.11g ones can be found for about $10 or so and even have an optical audio out.
You can get Airport Expresses on Goodwill for $5 to $10, but you need an old machine to set it up.
The current version of Airport Utility will see them in an extended network, and plot them on the network graph, but can't be used for configuration.
It's still there. I have an Airport Extreme that very happily updated itself to support Airplay 2 a few months again
But why wouldn't it work with modern routers? As far as I know, even the newest WiFi routers still support 802.11g clients.