How important is it for you to have the possibility of decapitation for any character unlucky enough to receive damage? What should the chance of a character being maimed be? Does this add to the lethality of a game or is it a step too far? How severe should the effects be? Does it depend on the system or setting?
For me, most of my answers would really depend on genre. For example, if I were getting into a supers game, I would want to know what era and what the "comics code" for the game would be. Fantasy and Sci-Fi could go in any direction, but I would expect horror or military genres to have a good amount.
In games, setting is informed by system, and unfortunately that doesn't always work out for the best. This kind of question, to me, points out some of the limitations of a HP-based system. It just doesn't spit out meaningful results for the fiction. I first noticed this back when I had to use a Troll. (1e, IIRC. Possibly Red Box Basic) The description made clear that they could regrow or reattach limbs, etc. I was immediately struck by how this can't happen under the HP system (and never had in the several years of game narration I had experienced so far.) I mean, you can't very well Claw-Claw-Bite someone when one of your claws has been lopped off, and there is no guidance for how many HP your arm is worth, or description of when your attack routine is disrupted. (The occasional monster back then had special rules for it, but I digress...) So, choose your system (or system options) wisely, I guess.
As far as lethality goes...well, if I take off your arm, you've just lost either a shield, a weapon attack, or the ability to use two-handed weapons (or climb or clap

). So that would add a level of "death spiral" to the game, if it wasn't there before. Certainly a bunch of maimed characters popping up would bring a level of grimness to a setting.