I have set up ‘Slide’ curtains in Home Assistant and exposed them to Google Assistant. In the Google Home app they now show up as blinds (instead of curtains). Google should be able to recognize curtains and the device class in Home Assistant is set to curtain (within Cover domain).
Mine still works indeed. At least last night it did.
I have it set up via the configuration.yaml. never bothered to move it to the UI set up.
Check if you slide app works via the original app first though. I did have some problems there some time ago. After contacting support they reversed an update which fixed my issues.