An un-safe supply of alcohol would be the sort of stuff that was common during prohibition, the sort of home brewed moonshine that poisoned people and made them blind.
We already do harm reduction and safe supply of many drugs in our society because we recognize the alternatives of adhoc illegal supply is dramatically worse.
Using opioids like heroin (boy if one could even get heroin at this point) is obviously not "safe."
These sort of concerns around safety are at this point however irrelevant because the reality is that people are chemically addicted to these harmful substances and the issue is how to best get them un-addicted or manage that addiction.
Anyone attempting to get off a chemical drug addiction can be absolutely be expected to relapse at some point (many points most likely!) along the journey to ultimately kicking the addiction for good.
The point of harm reduction is so that when someone relapses they do not die from street drugs that are so incredibly toxic there is no known safe dose and that they have yet another opportunity to try to wean themselves off their harmful addiction.
The context of this discussion around street drugs is about the unregulated street drugs that are so cut with various unknown chemicals that it is impossible to have a dose that yields any sort of expected results. This is what is resulting in an incredible spike in overdose deaths.