That and people have completely switched to MDMs to provide a lot of the old functionality that macOS Server used to provide.
The popular ones for macOS are Jamf and Mosyle. There are some other ones as well that are more cross-platform oriented, but both Jamf and Mosyle more or less cover the features that macOS Server provided for administration of a set of devices.
Tbh, I don't think Server was popular as a NAS service ever. The only thing I've ever used Server for was managing devices.
The popular ones for macOS are Jamf and Mosyle. There are some other ones as well that are more cross-platform oriented, but both Jamf and Mosyle more or less cover the features that macOS Server provided for administration of a set of devices.
Tbh, I don't think Server was popular as a NAS service ever. The only thing I've ever used Server for was managing devices.