Short summary: two major release branches are actively supported; shortly
after the (coming soon, hopefully) release of NetBSD 7.0 we will EOL
all netbsd-5 branches.
This is spelled out in all security advisories, for example:
http://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2015-003.txt.asc
which says:
Please note that NetBSD releases prior to 5.1 are no longer supported.
It is recommended that all users upgrade to a supported release.
Does this answer your questions? Let me know if you need more/other details.