Just like lead-based paint, they stopped selling the OLD GLASS when the code/law was changed...the old glass can't be replaced because the code changed and code enforcement is not going to care what glass was there when it was broken - they WILL care about what it is replaced with and the landlord doing it themselves to save the negligent tenant $$$ is not cost effective....had the tenant not broken the window, it could have lasted for years
as I said, I do not ever remember a time where non-tempered glass was used in sliding glass doors.
but regardless, if there were grandfather laws that allowed the continued use of the non-tempered glass, the remaining panes would still not have to be replaced now due to the grandfathering.
so, if there was no grandfathering, LL was required to replace the glass years ago when you suggest the code changed away from non-tempered glass. As such, the glass should have all been tempered anyway and the tenant breaking the glass did not cause the LL any damages, especially with the undamaged panes, simple because he was already required to replace them whether the tenant broke one or not.
anyway you look at it, the tenant is not liable for anything beyond the one pane, at most. A grandfather clause would still cover the unbroken glass.
that in itself gives reason to believe that there never was a grandfathering clause.
as to your paint analogy; ok, fine. when lead based paint was outlawed and not allowed to be used in a residence, the paint had to be mitigated regardless of the condition of the paint. Simple put, the lead based paint had to be removed or covered. So, lets use that analogy: LL had to replace the glass years ago when the code changed. He simply didn't. Tenant is not liable for the replacement. LL is, by law.