A quick search for “site:intel.com quartus version 15.1” returns a link to the "Product Discontinuance Notification - ADV 2011 - Discontinuance of Selected Intel® Quartus® II Software Subscription & Web Editions ":
“Intel® is announcing the discontinuance of all older versions of Intel® Quartus® II software Subscription Edition and Intel Quartus II software Web Edition released from 2002 through 2013 (except Intel Quartus II software Subscription / Lite Edition versions 13.1.4 and 13.0 SP1, which will remain available). The versions covered by this announcement include all variants of these versions that support different operating systems.”
My guess is that 13.1.4 and 13.0 SP1 are used by the military/government contractors, so it will probably be good for another 20-50 years 
I must have downloaded it just before they removed it.
EDIT: 2013 should not include 15.1 (2015), but I can not find a “Product Discontinuance Notification” that explicitly mentions “Quartus Prime Lite Edition Version 15.1.0 Build 185” or the year 2015. A search in the intel download center for fpga software gives the oldest version, excluding 13.0SP1/13.1.4, is currently “Intel® Quartus® Prime Lite Edition Design Software Version 17.0”. So Intel are probably deleting Quartus software that is older than 5 years.