Anything can be a server if it's set up to serve, so the distinction isn't useful here. Many people, educational organizations, companies, corporations, et cetera bought machines which have a desktop form factor which were used as servers.
https://en.wikipedia.org/wiki/SPARCstation
That being said, most orgs had little reservations about putting workstations into server roles, including every company I worked for, such as a regional ISP.
the "SPARCServer 20" was a super rare badging for like, workgroup servers
the big rackmount units for the sun4m era were still VME systems: 4/670, 4/690, etc