The question is predicated on a simple misconception about the meaning of "cut off". In fact, to be "cut from the people" simply means to be excluded from the congregation, ie, excommunicated, at least temporarily and in some cases permanently. Numerous hygiene laws required this as listed in places like: Ex 30:33, 38, 31:14, Lev 7:20, 21, 25, 27, 17:4, 9, 10, 18:29, 19:8, 20:3, 20:6, 17, 18, 22:3, 23:29, Num 4:18, etc, etc.
In almost all cases, including Lev 20:18, there is no suggestion of death or capital punishment.
The hygiene laws of Israel were part of what kept them relatively disease free-
Ex 15:26 - saying, “If you will diligently listen to the voice of the
LORD your God, and do that which is right in his eyes, and give ear to
his commandments and keep all his statutes, I will put none of the
diseases on you that I put on the Egyptians, for I am the LORD, your
healer.”
Now, part of the hygiene laws was what we now call "Quarantine" - isolation for a period of time to allow any infection to either show up or be destroyed, etc. It is also now known that blood is one of the most powerful vectors of diseases and rapidly spreads contagion. When this simple fact was realized in the late 19th century, the mortality rate among birthing mothers was drastically reduced after doctors washed after any blood contamination, especially uterine blood contamination.
Further, the foundational principles of Levitical law among the ancient Israelites were these:
- Prohibition of blood in the diet, Gen 9:4, Lev 3:17, 7:26, 27, 17:10-14, 19:26, Deut 12:16, 23, 24, 15:23. (see also Ps 50:13, Eze 33:25, Acts 15:20, 29, 21:25). The stated reason for this was the "life (literally "soul") was in the blood". Gen 9:4, Lev 17:11, 14, Deut 12:23, etc.
- Various hygiene regulations such as childbirth, bleeding, contamination and defecation, Lev 11:31-40, 12:1-5, Deut 23:12-14
- Regulations amounting to quarantine laws about contagious skin disease, Lev 13:1-46, 14:1-32, Num 5:1-4
- Regulations about molds and mildew, Lev 13:47-59, 14:33-57
- Quarantine laws concerning diseases of body discharges, Lev 15, Deut 23:9-11
- Care laws about handing dead bodies and carcasses, Lev 17:15, 16
- Prohibition of eating old (more than 2 days) meat, Lev 19:5-7
[There were also dietary laws but these were not part of the hygiene code.]
In modern times we recognize most of these regulations as simple hygiene cleanliness.