Definition
EOSL marks the date a vendor stops supporting a product with patches, updates, or fixes, including security fixes. For IBM i shops running older Power hardware or unsupported software versions, EOSL is a planning deadline, not just a warranty detail, because unpatched vulnerabilities and unsupported configurations accumulate risk the longer a system runs past that date.
EOSL planning is a separate question from whether an application needs to be rewritten. A shop can address EOSL on the hardware or OS layer, through options like PowerVS or a platform refresh, while leaving RPG, COBOL, and other application logic exactly as it is.