Deletion of Maintenance-free operating period

Comment: per  and lots of sources out there. XOR&#39;easter added a couple of sources to make this short stub well-referenced. A  style search quickly yields many GScholar and GBooks sources discussing the concept. Hence keep. --&#123;&#123;u&#124
Stance: keep
Cited policy: Wikipedia:The Heymann Standard (WP:HEY||WP:BEFORE)