en.wikipedia.org/wiki/Mount_Edziza_volcanic_complex

The infobox entry says the last eruption is "Unknown", but authoritative sources give a known late-Holocene eruptive age.

• The Smithsonian Global Volcanism Program volcano profile for Edziza lists "Last Known Eruption 950 CE" and notes that the complex had "felsic and basaltic eruptions as late about 1,000 years ago."
• BC Parks likewise says that after Mount Edziza became dormant, "numerous small eruptions" formed more than 30 cinder cones, and that these cones were formed "no more than 1,300 years ago."

So while the exact year may be uncertain, the last eruption is not unknown in the ordinary sense: reliable sources place eruptive activity in the late Holocene, roughly within the last 1,300 years, with Smithsonian using about 950 CE as the last known eruption date.