My understanding is those files for previous versions go away so you can't reliably get packages for older versions even if you "pin it". If I'm wrong, please let me know, I would like to be wrong.
I don't know what volunteer / manpower is needed to just keep files around but storage and bandwidth is definitely an issue.
I do agree that FreeBSD tends to delete stuff the second that it goes EOL. The ports project is the worst violator of this. The ports tree takes manpower to maintain and keep building. The mirrors for packages use very large donated systems and bandwidth (AFAIK) and don't have as much pressure to delete old packages.
I don't know what volunteer / manpower is needed to just keep files around but storage and bandwidth is definitely an issue.