The new dlc, Biotech, adds arenthood, and one of the mentalbreaks causes a pawn to leave the colony, that pawn can be a parent to one of your child pawns
There is slavery added with Ideology, and one of the mentalbreaks that can happen is called Sadistic Rage, that will cause the afflicted pawn to abuse your prisoners, that pawn can be a slave
Biotech adds children, and thete is a setting in the game start that can be turnt on, when enabled it allows children to become and be part of raids on your colony, which you will have to use force to stop
You can butcher animals, including pets, and starve them deliberately. With certain mods, you could even lobotomize them, though the player can choose to not install those mods.
Dogs can fight eachother in several circumstances (two wild dogs battling for food, a tamed dog fighting a wild dog, and a tamed dog battling one that's infected with scaria).
There is a chance that your colonist will spawn with the "Music Idol" childhood backstory, which specifically states that when your colonist's parent died, your colonist "could not bear their parent's loneliness, and became his/her sexual companion."
Aside from this, there is none. It does not happen in game at all, unless you count adopting a random child from the wilderness, raising them as a parental figure, and dating them when they turn of age. That does happen sometimes.
you can use the 'euthanize by cut' operation, which slices the throat of a pawn. in addition, attacks can hit pawn's necks, though it isn't very graphic
decapitation can happen from a variety of causes. Additionally, players can order one of their colonists to extract the skull from a corpse, effectively decapitation them
If you mine too far under a mountain and don't put up supports for the roof, there's a chance your colonist will get crushed when it collapses. It is a very quick process, and they kind of just poof off the screen though. It's not graphic at all.
When raiding a faction's base, if you choose to, there will sometimes be a baby which you can adopt, raiders who attack your colony can also kidnap babies and children too
Biotech adds children, and when a setting is enabled, they can and will become raiders who can attack your colony, you will need to kill them as there is no reasoning with any raiders in the game
Biotech adds children's toys that can be built, a pawn can have a mental break called Tantrum, and can choose to break a specific thing, or just attack different structures, one of the things that can potentially be destroyed is the child toys you build
There is cannibalism in the game, but the human who is eaten is not alive, and animals can also eat corpses, but will attack a human until they die and then eat them
There aren't mechanics in game for trans people, but there are pawns with pregenerated backstories, and the only one of these that is trans is just a regular jeweler.
There are player-submitted pawns that have custom backstories, and the backstory for Alyssa 'Sparkles' Orchard reveals that she's transgender and contains her birth name. She cannot otherwise be deadnamed unless you deliberately change her nickname to it, and there are no mechanics that allow for other characters to have deadnames at all.
Body purists, who hate the idea of artificial body parts, can be given them anyway. They very much despise this.
Conversely, body modders and transhumanists have some dysmorphia if they have no artificial body parts.
Technically it's possible - character can self-harm and bleed out, but self-harm is rare issue and in most cases it's possible to help self-harming character. Characters can also perish in explosions caused by themselves.
Pawns below 16 can't get into relationships, and it's hard for another couple years, especially with large age gaps, but it remains possible. I have a save where a 16 year old got into a relationship with a 43 year old.
You can have naked colonists, and some prefer to be naked, but the lack of detail in pawns means that you only see skin-colored blobs. (And all skin-colored blobs are not necessarily naked, as pants do not show up on pawn models.)
Sex is exclusively referred to as lovin' and it is not depicted at all besides putting some floating hearts around the shared bed where lovin' is happening.
There is an event in the game called Wild Person, where they are essentially treated like an animal by the game, and they live outside your base in the dirt