This is something I've responded to quite a few times on here, I know it's not common knowledge and I only found out myself from experience and research. In many countries, the U.S. for example, the laws and policies that govern involuntary hospitalization and treatment for mental health are completely separate from criminal law.
Criminal law affords the suspect a bunch of rights; the police need a search warrant to enter your home, you have the right to stay silent or get legal representation before you answer any police questions, and many other things. "Mental health statutes" vary by state in the U.S., but many allow a single police officer to forgo any of the rights that a suspected criminal would get, and make the determination by themselves of if you "appear to be a danger to yourself". Here's an example of
Colorado's statute.
Many things a person would use to CTB aren't banned or illegal, and in the U.S it's not even illegal to kill yourself. So the police aren't acting on criminal law whatsoever, it's a completely separate area of law.
This example we're discussing, a guillotine, is well into the realm of what a police officer would likely use to say you're a danger to yourself. They often use much more obscure things, and they don't have to see any physical evidence, they can make the determination off of if you "appear" to be a danger to yourself, which is very vague and unclear wording. So yeah, a literal guillotine that someone can see as physical evidence is
way more of a risk than would be sensible to take.
Again, this varies by country, and even has small variations state-by-state in the U.S., with some countries tending to be significantly worse than others. But what many countries have in common is that police interactions for mental health fall under a separate area of law from criminal law, and thus, what's banned or illegal is irrelevant.