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Answer
Most Taiwan price datasets silently drop companies once they stop trading, which inflates backtest returns through survivorship bias. TW Market Data keeps the full daily price history of the TWSE universe — 1,682 stocks and 5.2 million daily rows since 2004-02-11 — including 311 stocks that have since stopped trading (delisted or long-suspended), 262 of them with an official delisting date. So your universe as-of any past date reflects what was actually tradable then, not just today's survivors.
Survivorship bias occurs when a dataset silently drops companies that stopped trading, so a backtest only sees today's survivors. That inflates returns and understates risk, because the failures that would have hurt the strategy are missing from the historical universe.
Reconstructing the universe as-of a past date requires the price history of stocks that later stopped trading, plus a last-trading-date / delisting reference, so the universe reflects what was actually tradable then rather than what still trades today.
The TWSE daily price series retains 311 stocks that have stopped trading (delisted or long-suspended for more than 90 days), 262 of them carrying an official delisting date. Coverage runs from 2004-02-11, and TPEx daily prices go back to 1994-01-05, so you can reconstruct a point-in-time universe rather than only today's survivors.
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