Debian (especially the main branch, 'stable') is as the name implies highly stable, secure though? Definitely not! Outdated kernel, outdated libraries, very long release cycle: you can't backport every security fix for 3 years while the rest of the ecosystem is rapidly progressing.

Industry wants Long Term Support, but rolling release (or for increased stability 'short' (< 1 year) release cycles) are the way to go when it comes to security.