There is a specific answer to this, and it surprises people, because it means leaving something in place that looks exactly like the thing debridement removes.
In general, debridement may not be appropriate for dry and intact eschars with no clinical evidence of underlying infection. One example is an unstageable pressure ulcer with an intact eschar at the sacrum, buttock, or heel.
Eschar is the hard, dry, dark layer that forms over some wounds. The reason for leaving an intact one alone is simple: in these cases, the intact eschar serves as a biological covering over the underlying skin defect. It is doing the job a dressing would otherwise be doing.
Both conditions matter, and they go together. Dry and intact. And no clinical evidence of underlying infection. If either one doesn’t hold, this is not the situation being described.
That’s why a dark, hard area on a heel may be left on purpose while another wound is debrided, and it’s worth knowing before you decide one wound is being neglected. It’s also why this isn’t a judgment to make at home. Whether there is evidence of infection underneath is the whole question, and it’s answered by examination.
Other reasons not to use a method apply to each particular type of debridement, so a wound that isn’t suitable for one approach may still be suitable for another. Deciding between them is the clinical work.
You should never try to do debridement yourself. That holds in both directions: neither removing nor deciding to leave is a home decision.