Apple quietly keeps an El Capitan installer alive for owners of legacy hardware. Visit this official support article:
Recovery and Internet Recovery: Macs that shipped with El Capitan (or earlier) can often reinstall their original OS via Internet Recovery (hold Command+Option+R at startup) which downloads an Apple-signed installer directly. mac os x 1011 el capitan iso download link
Integrated directly into the system files. Apple quietly keeps an El Capitan installer alive
Released in 2015, remains a vital operating system for users of older Mac hardware. Named after the iconic rock formation in Yosemite National Park, El Capitan focused on "refining the experience and improving performance" over its predecessor, Yosemite. mac os x 1011 el capitan iso download link