For five years, PG&E Corp. repeatedly delayed a safety overhaul of a century-old high-voltage transmission line that is a prime suspect behind the deadliest wildfire in California history.

The company told federal regulators in 2013 it planned to replace many of the towers, wires and hardware pieces on the line, called the Caribou-Palermo, regulatory filings show. It again proposed the project in 2014, 2015 and 2016—pushing it back each year. The company planned to start work June 2018 and finish late last year. It hasn’t...