Say you install FreeBSD 12.1, you can keep your package system using the 12.1 package set forever. No updates will be available. But they'll always work.
In /etc/pkg/FreeBSD.conf you will find something like this:
> url: "pkg+http://pkg.FreeBSD.org/${ABI}/quarterly",
Assuming using amd64 that maps to:
https://pkg.freebsd.org/FreeBSD:12:amd64/
There you can see there are "release_X" sets that map to 12.0 12.1 12.2 12.3. Again, no updates will ever be published on those but they will be guaranteed to continue working so long as they don't get deleted.
The FreeBSD community just doesn't have the manpower (of volunteers) or willpower to maintain what you're asking for.
My understanding is those files for previous versions go away so you can't reliably get packages for older versions even if you "pin it". If I'm wrong, please let me know, I would like to be wrong.
I don't know what volunteer / manpower is needed to just keep files around but storage and bandwidth is definitely an issue.
I do agree that FreeBSD tends to delete stuff the second that it goes EOL. The ports project is the worst violator of this. The ports tree takes manpower to maintain and keep building. The mirrors for packages use very large donated systems and bandwidth (AFAIK) and don't have as much pressure to delete old packages.
In /etc/pkg/FreeBSD.conf you will find something like this: > url: "pkg+http://pkg.FreeBSD.org/${ABI}/quarterly", Assuming using amd64 that maps to: https://pkg.freebsd.org/FreeBSD:12:amd64/ There you can see there are "release_X" sets that map to 12.0 12.1 12.2 12.3. Again, no updates will ever be published on those but they will be guaranteed to continue working so long as they don't get deleted.
The FreeBSD community just doesn't have the manpower (of volunteers) or willpower to maintain what you're asking for.