Prominence of less-lethal weapon (also called less-then-lethal as well) in self-defense applications by civilians are on the raise in the later years. The advertisements are abundant. Those are many good reasons for this and this trend is in-line with raise of less-lethal and (so called) non-lethal weapons in standard use by Western law enforcement and military personal. In general human society became less receptive to physical torture and murder ether of humans or animals that at any other time of our history, even our brutality still mind shattering.
In the list of civilian self-defense weapons that considered less-lethal or (sometimes) non-lethal two distinct categories can be seen:
The important point to understand is that it's no such thing as truly non-lethal weapon and this term is just a politically expedient misnomer created to placate general public.
Any weapon effectiveness based on applying physical or physiological damage to its target. This damage is differ in severity and reversibility. But any physical or physiological damage to a human being can be potentially lethal. Numerous cases of death from taser application promoted many police departments to putt their policies of its use under review (and even implement moratoriums on its use, dispute all its advantages).
This definition of non-lethal and less-lethal weapon addressing wide range of devises in police and military (when military is acting as a peace force) use. It going all the way from traditional and collapsible batons to low-frequency targeted acoustic beamers. And remember that any of them can potentially cause irreversible harm or death (and risk, in many cases, is depends on subject's health conditions that you not likely to know beforehand).
Use of any weapon requiters training. In civilian self-defense class use of "low-tech" blind impact less-lethal weapons should be thoroughly addressed. Especially - in my opinion - the use of collapsible batons and brass knuckles.
Also, in some jurisdictions brass knuckles can be legally seen as deadly weapon and prohibited for care the same way as knifes (bladed weapons). The effect of hits using modern collapsible batons, which in open configuration have flexible metal pole with a short heavy cylinder at the end, and traditional brass knuckles can range from relatively mild, when applied to limbs muscles, to deadly, when applied to some vital points or to a head (even not directly to head vital points).