Is the 2015 Sazerac 18 the last of the original tanked batch?
Yes, and Buffalo Trace said so directly. Its 2015 release notes describe this as the last edition of Sazerac 18 from the stainless steel tank that had held the spring-1985 rye since 2003. Every Sazerac 18 released from 2016 onward comes from different whiskey.
Why was the old Sazerac 18 held in steel tanks?
The spring-1985 rye reached eighteen years in oak in 2003. Rather than bottle it all at once or leave it in barrel to grow more tannic, Buffalo Trace emptied the barrels into a stainless steel tank, which stops maturation because steel adds no wood character. The distillery then drew from that tank once a year for the Antique Collection until the 2015 release emptied it.
What replaced the tanked stock after the 2015 Sazerac 18?
Buffalo Trace stated that the 2016 Sazerac 18 would be made from whiskey aging in barrels filled in April 1998, with later releases drawing on 1999 fills and so on. So from 2016 the release comes from rye that stayed in wood for its full eighteen years rather than from the 1985 tank.