Borgmon and Borg are different things, but yes, before the latter, machines were owned by teams. Brian Grant talks a bit about the predecessors, Babysitter and GWQ: https://softwareengineeringdaily.com/2018/04/27/google-clust...

Borg was started in 2003 and by 2006-2007 was already the default way to run things, even though isolation wasn't perfect then. I think it started with chroot jails, then fake NUMA, then cgroups, which were written for it. It took years before all of web search moved to dedicated Borg machines (their quota belonged to you and nobody else could run on them) and, eventually, shared ones.