WP. There have been two underlying problems for defined-benefit corporate pension funds. The first is that the companies want nothing to do with them, as they increase the volatility of profits and were underfunded for many years, often requiring an injection of money. The second is that successive governments have used pensions as a piggy bank.
In between scrapping tax relief on dividends in 1997 and announcing a change in 2020 to the inflation calculation for inflation-linked bonds to a method that suppressed reported inflation numbers, the government was very happy to stuff savers, pension funds included, with quantitative easing. The result was that lower long-term interest rates massively increased the current value of pension fund liabilities.