Your idea of "proven-safe" is that they were used for two decades without proof of problems. Those examples fit that standard.
The standard should be comparing outcomes in users and non-users after 20 years. The idea is that 20 years is enough to show any effect that exists, and we shouldn't refrain from adopting the new thing waiting for even more evidence to come in.
Cigarettes, asbestos and leaded water would all show strong effects after 20 years, not as strong as after 50 years, but certainly enough to identify a problem.
Certainly some things have a very delayed effect, but they are very very few compared to things that have a quick effect; most foods that are bad for you show...
Basically: How does one pursue the truth when direct engagement with evidence is infeasible?
I came to this question while discussing GMO labeling. In this case I am obviously not in a position to experiment for myself, but furthermore: I do not have the time to build up the bank of background understanding to engage vigorously with the study results themselves. I can look at them with a decent secondary education's understanding of experimental method, genetics, and biology, but that is the extent of it.
In this situation I usually find myself reduced to weighing the proclamations of authorities: