Not for the Chaos Heretics
May the Emperor have mercy on your Soul for His servants cannot. - Inquisitor Thaddeus
I assume by "heretic" you mean "someone who has forsaken the Emperor to some extent for Chaos." This is a specific type of Heretic. It's also heretical to have dealings with proscribed xenos, to be in contact/know about certain xenos, to be in contact with any xenos outside of very rare circumstances, to Rebel against the Imperium for any reason whatsoever, to worship the Emperor or any of his saints in a way unsanctioned by the Ministorum etc. For these there are punishments other than execution. Normally still execution, but also enslavement and "re-education" that's normally just means "worked to death whilst being preached at." However Chaos is "The Archenemy" of mankind, and a chaos heretic is a different story.
The nature of the Warp means that there is always the chance that a person touched by the warp, even one who is honestly trying to repent, will backslide into heresy or worse (possession, etc). It would be a failure of duty to allow such a person to live, outside of very specific circumstances none of which end with "let off with a lecture." Even if they don't fully comprehend that what they're saying/doing is heretical. To do otherwise is not only imperiling the ex-heretic's soul (because of the penchant to backslide) but also a physical and moral danger to the people around them. The ex-heretic might revert, and convert others. Or try hard not to revert, but Chaos still has it's talons in their mind and they become a psyker and open a warp rift that dooms the whole planet. It's not a risk the Inquisition is usually willing to take.
In Very Specific Instances the heretic is intentionally kept alive, but always with a death sentence in place, just not carried out. For example in Gaunt's Ghosts Mabbon Etogaur (probable former Imperial Guardsman and actively a Chaos General) was allowed to live because of the information he contained. It couldn't be extracted by torture, and he was essentially treated like a "normal" high-security prisoner. Albeit with strong overtones of "this guy is Chaos filth and might be able to do witchcraft." In other instances high-ranking heretics are kept live by more radical inquisitors to plumb their minds for warp-secrets, or by puritans so they can be tortured "sufficiently" to atone for their sins. Low-level heretics (what you might call "didn't know what they were doing" types) are sometimes used as slave labor in situations that will quickly kill them. But more often they're turned into servitors or arco-flagellents or whatever. Since all of these lobotomize you at absolute best they're basically death sentences as well.
That "do they let heretics repent" aspect of your question is super interesting. Heretics in are ALWAYS given a chance to repent, unless it's a battlefield situation. (and sometimes even then! though being shouted at to repent whilst being shot at seems... unlikely to get a positive result.) The "problem" is that repentance doesn't save them from execution. At best it's the difference between a quick execution or a long tortured-to-death style end. In the Imperial Creed there is no repentance in this life that will actively "save" your soul once you've been touched by the warp. As only the Emperor can grant you absolution, the Inquisition's job is to arrange the meeting. How much punishment the Inquisition gives you before you face the Emperor's judgement depends on the level of your sins and your level of remorse. Again a Very Important Heretic might repent and be allowed to live, but only because they are more valuable to the inquisition alive than they are dead for whatever reason. Mabbon Etogaur is again a great example of this. He seems to be actively trying to repent the entire time he's in custody. But I won't go deeper in because the spoilers aren't super pertinent to your question.
As a sidenote on penal battalions there IS a method of Parole. The famous "last chancers" 13th Penal Legion being an example of "last one alive gets the Emperor's forgiveness" style parole for Penal Legion troops. For some minor offences surviving X number of engagements can also be a way out, but "X" tends to be "statistically impossible for you to have survived that long" so it's more a panacea for the legionaries than a thing the Imperium ever things it'll have to actually honor. Also as it relates to your question "heresy" is never considered a "minor offence" so a heretic wouldn't get out of a Penal Battalion that way.