Small, old fashioned basement refrigerator that requires periodic
defrosting. The fridge has worked great for 25 yrs. but almost from
the beginning, the light inside flickers and then goes out, or else
doesn't even flicker, just stays dark inside when the door is opened.

The other day an electrician fixed a vestibule light that flickered.
He made his fix in the wall switch, and said that it could have been
dangerous if left uncorrected. I forgot to mention to him the
refrigerator light problem.

Is this the same type of electrical situation? The fridge kicks on
immediately when plugged in after defrosting and works great; it's
just the light that is the problem. Thank you.

It is only dangerous when it flickers. If it stays lit or stays dark, it's
not dangerous.

This is because flickering light is often caused by a poor contact between
two conductors, and this area of poor contact could generate heat that over
time may develop into a fire.

If after 25 years nothing has happened, then nothing is probably going to
happen. But just for the peace of mind, it's better if you fix it. If you
don't want to, then remove the bulb.