In all the cases I have seen, the contributions were recovered under the 3-Year Rule (probably before your time) and the pension is now fully taxable. I think if retirement was after the new rules became effective, they figured the annual exclusion amount and it shows on the 1099. And often, that has run out, because the costs were recovered over a period of 15, 20 or 25 years. Survivor annuities could be more complicated.
And then there were the people who retired on disability -- their pension counted as sick pay, or something like that, until they reached normal retirement age. Sometimes the 1099 continued to show no amount in the "Taxable Amount" box.