The Scorn are zombified, mutated versions of the Fallen. Additionally, the lore from Beyond Light regarding the Exo project has elements of body horror as it details the incidents with the first artificial bodies, particularly extra limbs being attached to a test subject.
Not elaborated on in-game, but there are several pieces of lore that detail Calus's abuse toward his daughter Caiatl. The conflict between the two characters ultimately stems from this abuse and is a major theme in Season of the Haunted and Lightfall.
Due to the nature of Guardian immortality, there are some couples who have decades or centuries of age difference (Osiris and Saint-14, Zavala and Safiyah, Ana Bray and Camrin Dumuzi). However, all parties involved are mentally and physically adults.
Banshee-44, the gunsmith in the tower who you will visit frequently, suffers from memory loss. He often repeats things he just said and always seems disoriented.
The lore from Beyond Light talks about Dissociative Exomind Rejection (DER), caused by Exos realizing they are in an artificial body instead of a natural one.
You can stab enemies using glaives or Monte Carlo's bayonet, but these are both secondary features of their respective weapons and stabbing is never required.
Every mission from Season of the Deep, including the Ghosts of the Deep dungeon, has an underwater section (technically it's under methane because it takes place on Titan, but functionally its an underwater section).
Yes, there are seas in the EDZ and Cosmodrome, the methane ocean on Titan, the swamp in Savathûn's Throne World, and some sort of Neptunian fluid surrounding Límíng Harbor on Neomuna.
Any content featuring Savathûn or the Witness almost always includes some amount of gaslighting or manipulation, most notably the Witch Queen and Lightfall DLCs.
No actual snakes, but a lot of snake imagery, especially surrounding the Drifter and his gamemode Gambit, the Veist weapon foundry (which names some of its weapons after snakes), as well as Hunters and their armor (snakes are their class symbol).
During the Forsaken campaign, a character struggles to breathe as he dies. However, this character was an Exo (a sort of robotic human), and doesn't need to breathe in order to survive. The sound of him struggling to breathe is still present, though.
So there's an alien species called the Fallen, who have four arms, but low status Fallen are "docked" and have only two arms. At least two NPCs of this species have lost an arm, and are either visibly without it, or have a cybernetic replacement.
At the time of writing there is a flashback cutscene accessible in-game where the 17-year old son of a character is killed, depicted in stylized artwork.
After the realease of the "Lightfall" expansion this cutscene will no longer be in the game.
Some storylines mention sacrifice being a tradition amongst Guardians. [SPOILERS] In the Lightfall DLC, one of the major NPCs (Rohan) sacrifices himself to help the Guardian continue their mission. His death is treated with honor, and there's a cutscene of his funeral. In Season of Defiance, a major NPC (Holliday) stays behind to make sure everyone escapes and is killed in an explosion. You attend her funeral as part of the quest.
In the game's lorebooks, there are examples of parents & parental figures dying. However, lorebooks are entirely optional and you do not have to read them.
One NPC/Vender, Failsafe, constantly switches between a cheerful personality and a pessimistic one. This is caused by damage to her internal processors and she's never stated to have DID. However, one now-unplayable mission has Cayde refer to her as "Failsafe and her evil twin, Failsafe", which could come across as the "evil alter" stereotype, although both personalities are good.
[SPOILERS?] Crow has a brief anxiety attack in Season of the Risen after accidentally killing Caiatl's Psion & in Season of the Haunted after failing his first Severance, but they aren't too graphic.
A Playstation-exclusive/possibly cut line in the strike Inverted Spire has Ghost begin laughing hysterically out of stress and briefly sound out of breath while talking to Zavala, but it's audio-only and not taken seriously.
Not exactly but one of the main villains, Eramis, is implied to struggle with suicidal thoughts. One lorebook has her wonder if the Fallen would be better without her, and tries to goad Mithrax into killing her at the end of Season of Plunder.
From now inaccessible missions: 1. In the Red War storyline, Ghost has a rare chance of saying "Twinkle twinkle, assh*les!" after the Guardian gets their Light back, and 2. In the Forsaken storyline, Ikora calls Uldren a "son of a b*tch" after Cayde-6 is murdered.
Elsie refers to Clovis as a "b*st*rd" during certain Beyond Light missions.
Unintentional; the Cabal share a name with the antisemitic "cabal" conspiracy, and one season (Season of the Chosen) contained the phrase "Blood and Honor" & had you collect their gold. Bungie apologized for this and remarked it was not their intent, then patched the phrase out.
As of the Lightfall DLC, one main character is nonbinary and is always referred to by their preferred pronouns. Some pages of the "Last Days" lorebook also have nonbinary characters who are referred to by their preferred pronouns.
You can make your speeder crash and explode spectacularly, usually after falling from a great height, but it can also take too much damage and explode. The speeder will always eject the player before detonation, but the explosion may kill the player anyway.
There are crashed planes in the Cosmodrome, and crashed spaceships on the Moon, Tangled Shore, and Europa, but the crashes themselves are not shown. In Season of Defiance it is mentioned that some pilots crashed after a space battle, but this is again not shown.