Is Airplay dead to Apple? They don't seem to support streaming music anymore.
Apple does support Airplay but only to their own speakers like airpod. Or Apple TV.
They removed the Airport Express from the line up.. so you will find it more difficult to use your old HiFi system and so be forced to buy Apple branded speakers.
Most of the big sound companies do provide airplay in Amplifiers now so there is still support at least on higher end gear.
BTW Belkin has just released an Express replacement though I have seen few details of it.
https://homekitnews.com/2021/05/30/belkin-soundform-connect-with-apple-airplay-2/
The problem with most routers is they lack support for Bonjour protocol. That does not mean Airplay won't work but it is a matter of trial and error. My advice is just to do a google search using the router model and airplay or bonjour terms. See if there is a general issue amongst the community.
Router manufacturers really care very little if Apple protocols work properly and you will get amusingly wrong advice from most of them who clearly have no idea about it. And I am talking big companies like Netgear.. not just backwater little Chinese manufacturers.
My old Airport Extreme router doesn't support secure wifi. I have four Airport Express units for streaming music. If I buy a new secure non-Apple router, will it still play Airplay to my Airport Express units?
If you take some care in the setup this can and will usually work. It might mean you need to setup one Express as a DHCP device in the network. Post back if you need more details.
Also note if you buy WPA3 router the Express will have to connect by ethernet. Since it can only do WPA2 security.