Depending on your specific definition, Redemption denotes the idea of clearing oneself of wrongdoing or the guilt of wrongdoing. However, it is not possible to “clear” oneself of wrongdoing. Nothing one does will ever make someone happy/OK with that their mother was heinously killed, nor will it absolve one of the guilt. Even if you go on to save another 100 lives later in life, the harm you did was unnecessary and cannot be redeemed away (This, by the way, counts for every clear case of morally wrong conduct, even minor things). You will never be able to say that a bad thing one did was “redeemed” by anything you did thereafter. We don't have a moral account where you can transact good deeds onto and bad deeds away from - where you could do some many good things to “justify” a bad thing. Redemption like this exists purely in the realm of theology.

“Salvagability” and “redeemability”, ie. “redemption”, are not philosophical terms, they're theistic and rely on theistic principles. It is hard to establish some kind of moral “debt” to life or existence because, well, what authority are you basing that on? It can be “turned” into a philosophical term by further defining it using philosophical ideas.

On the other end, if you consequentialistically define “irredeemable” as “this person will never be able to do more good in their life than they have done bad” (which are value judgements) then this calculation is impossible to make for any one person until their death, because anything can happen to anyone. We don't know what good deeds they might still pull off before their end. And even still, if a person did something really bad and stands to die having done more bad than good, and they end up saving a passenger plane in their dying breaths then this doesn't mean very much. If you could save a passenger plane from crashing then anyone would do that, not necessarily because you're a good person but because you want to save yourself and just so happen to save other passengers on that plane. We care about state of mind: Has their moral compass improved, are they saving the plane out of compassion or out of the goodness of their heart? A person who committed heinous murder might have had a broken moral compass but might realize later on and change - redemption. Redemption in this sense is a progression from a broken moral compass to one that is fit, and this can happen spontaneously at any time, even to people who have done some of the worst crimes imaginable.

Humans improving, “fixing” their moral compass is why the penal system is originally meant to focus strongly on rehabilitation ¹⁾. People should be imprisoned if they pose a threat to other people (amongst other reasons), but if their moral compass improves then even perpretrators of some of the worst crimes imaginable should not be imprisoned for any longer than necessary to satisfy the pillars of sentencing ²⁾. Every human deserves to live the best possible life they can possibly have; to say that perpetrators of serious crimes deserve less even if they have underwent major change and the pillars of sentencing are satisfied would be immoral in my eyes.

Ultimately, “salvage” and “redeem” (redemption) are all terms used by victims to describe their (in-)ability for forgiveness. Their ability to relinquish resentment or some other form of morally inflected anger ³⁾. When we talk about someone who is irredeemable, the easiest definition after all is still “I personally will never be able to forgive that person”, but this is inherently subjective and will never not be subjective. Generalizing this into “this person is irredeemable by any human on earth” just doesn't work (without making concessions like “most people on earth”).