Famously, in 2008, Lehman Bros. was allowed to go out of business, when many REITs were being falsely evaluated as worthless, although some thought it provided near-essential services to the financial industry. It's ticker had been LEH. When I do a search in data finder or in SimFin Fuse, on either "LEH" or "Lehman", I get no hits.
As the point of this project is to provide some free data including fundamentals for backtesting, backtesting without these "dead" or de-listed tickers suffers from "suvivorship bias". This causes many strategies to look too good, since the stocks the backtests test over have all survived to the present day. Ones that have gone out of business have dropped out of the indices and virtually all free data sources. That's why CSI gives away cheap data through Yahoo: it could make a fine stock screener, if you already have a strategy, but how are you going to backtest it?
Thus, I would be interested to know if SimFin will be supporting dead tickers, or if it is intending to be just another free source of bad data figuratively blowing smoke up our collective asses?