I'm sure someone has, but has anyone made a list of killings in the back-stories of episodes? Ones that might or might not be actual murders, or might not get solved even if they are?

It isn't a long list, but these are the ones I can name.

Eric Mason's wife in How To Dial A Murder.
(Even if everyone is probably certain it was murder.)

Ruth's brother-in-law in An Old-Fashioned Murder.

Phyllis in Try And Catch Me.

Janice Caldwell in A Friend In Deed.
(It's possible that Hugh's story to Mark is entirely true, but you never find out.)

Rodger Stanford's parents in Short Fuse.
(That one seems pretty iffy, but I've heard at least one person here wonder whether their accident was his doing.)

Hmm, there's definitely Nadia Donner in "A deadly state of mind", the murder by hypnosis, and also Harry Alexander in "A stitch in crime".

Lisa Chambers' murder from "Double Shock" doesn't get resolved on screen. Also Charles Clay's murder from "Last Salute".

Both murders may have been solved later, off screen or maybe Columbo got confessions. It was either Dexter or Norman - or both - who tossed Lisa over the balcony of her apartment. And Charles Clay would have been killed by 'ol Tisn't himself as well.