Is there a law in Ontario that automatically wipes out very old, unused registered easements?

No — there's no blanket rule that erases a validly registered easement simply because it's old or hasn't been used in a long time. A common source of confusion is Ontario's older Registry system, where title searches have traditionally covered a defined historical search period as a practice for confirming clean title, but that search convention is about how far back a lawyer typically looks when reviewing title, not a rule that automatically extinguishes older, still-valid registered interests like easements sitting further back in the chain of title.

An easement that's properly registered and was validly created doesn't simply expire or vanish with age on its own. As covered elsewhere, extinguishing an easement generally requires something specific to actually happen, such as merger of ownership, clear evidence of abandonment rather than just non-use, or a formal release, rather than the mere passage of time or predating a particular search window.

If you're told an old easement "doesn't count anymore" simply because of its age, that's a misunderstanding worth double-checking with a lawyer and a proper title search, rather than assuming an old registration has quietly stopped applying.