The three Airport Expresses I had were the 1st-generation models. At the time Apple discontinued them, I had one of the 2004 WiFi 802.11g only and two of the updated n's released in 2008 and sold new through June, 2012. I had purchased more and given those to family.Just to second this, my Airport Express also works fine for streaming and music with a 2015 iMac 5k, Airport Utility 6.3.9, and Mojave 10.14.3.
I believe I was running Mavericks when I decided to change the password on one of my n's and Airport Utility had updated to 6.3.1, which was released July 11, 2013. I just presumed the inability to "find" the device was a device fail, which had happened before, and Apple had swapped a replacement as it was tied to a Mac purchase with AppleCare. Was able to change the password with a laptop still running Snow Leopard and Airport Utility 5.6.1
The 2nd-Generation Express continues to receive updates, though not at an encouragingly frequent rate. The most recent security update is 7.6.9 from December, 2017. Information at this Apple support article:
In August, 2018 Apple issued Firmware Update 7.8, which added AirPlay 2 support for the 2nd-Generation Airport Express.
Yes, it is possible to "manage" the old devices by getting Airport Utility 5.6.1 to run on a current OS. But they haven't had any security updates since 5.6.1 in 2012, which is why the ones I own are not in service.