It’s a tad harder, since they can be seen in your Sonos controller. And Sonos programmers wouldn’t have access to Spotify’s code, which would make it even more difficult for Sonos. I’d try, with no guarantee, a network refresh, on the offhand chance there is something in the network itself keeping Spotify from ‘seeing’ the Sonos.
Unplug all Sonos devices from power. Reboot both your router, and the device that you’re trying to connect from Spotify on. Wait two minutes after rebooting both of them, then plug in your Sonos devices. Wait another two minutes, then open Spotify on your device, and see if it can ‘see’ the Sonos again. If not, I’d suggest getting in touch with Spotify, not Sonos. It is entirely possible they made some change that is keeping your Spotify app from ‘seeing’ the Sonos system.