In TFS 2015 the "publish" task in a build definition may either push to the TFS server or to a fileshare/UNC path. Unfortunately, deleted builds aren't automatically pruned if you're using a fileshare. However, I'm unable to determine where TFS hosts the published builds when you're using the "server" option.
I do not have direct access to the TFS server, just the agent hosts. However, I have a sandbox environment whose local TFS admin doesn't appear to expose any configuration for where this path might be, either.
Does anyone have more information than I do? Where are these builds hosted in TFS? Where is this configured?