In response to a comedic tweet about pirating indie games using Ultrakill as an example, the game's creator Arsi "Hakita" Patala expressed support for piracy among those in situations like poverty. He noted that Ultrakill only came to be thanks to inspiration from works that he pirated in his youth, and encouraged supporting a game via word of mouth as an acceptable substitute for a purchase, saying: "You should support indies if you can, but culture shouldn't exist only for those who can afford it."

On December 7, 2022, the game's official Twitter account announced the release of UKButt, an official mod which adds overall controller rumble support to Ultrakill by running buttplug.io, an open-source program that allows select video games to interface with rumble motors in Bluetooth-connected devices, and is especially designed and marketed for use with adult toys. The announcement described the mod's release as the "Sex Update," referencing a longtime meme among the game's fanbase which jokingly demanded that Hakita "add sex to Ultrakill," to the point of leaving tongue-in-cheek negative reviews on Steam.

Many of the game's equipment items, enemies, and levels are named after popular music:

• The Feedbacker is named after the 2003 album by Japanese drone metal band Boris at Last.
• Whiplash is named after the 1983 song of the same name by American thrash metal band Metallica.
• Schism is named after the 2001 song by American progressive metal band Tool.
• Streetcleaner is named after the 1989 album by English industrial metal band Godflesh.
• In "Overture: The Mouth to Hell", V1's bootup sequence dates their firmware to the year 2112, referencing the 1976 album by Canadian progressive rock band Rush.
• "1-1: Heart of the Sunrise" is named after the 1971 song by English progressive rock band Yes.
• "1-2: The Burning World" is named after the 1989 album by American experimental rock band Swans.
• "1-3: Halls of Sacred Remains" plays on the song "House of Sacred Remains" from the soundtrack to Castlevania: Lament of Innocence.
• "1-4: Clair de Lune" is named after the third movement from French impressionist composer Claude Debussy's Suite bergamasque. "4-3: Clair de Soleil" further plays on the song, replacing "lune" (moon) with "soleil" (sun).
• "2-1: Bridgeburner" is named after the 2015 song by American heavy metal band Mutoid Man.
• "2-2: Death at 20,000 Volts" is named after the Tokyo Nightclub 20000V.
• "2-3: Sheer Heart Attack" is named after the 1974 album and 1977 song by English glam rock group Queen.
• "2-4: Court of the Corpse King" is a play on the 1969 song "The Court of the Crimson King" by English progressive rock group King Crimson.
• "2-5: All-Imperfect Love Song" is named after the 2002 song by Japanese electronic musician World's End Girlfriend.
• "3-1: Belly of the Beast" is named after the 2014 song by English experimental musician Gazelle Twin.
• "3-2: In the Flesh" is named after the 1979 song by English progressive rock band Pink Floyd.
• "4-2: God Damn the Sun" is named after the 1989 Swans song.
• "4-3: A Shot in the Dark" is named after the 1986 song "Shot in the Dark" by English heavy metal musician Ozzy Osbourne.
• "5-1: In the Wake of Poseidon" is named after the 1970 King Crimson album and its title track.
• "5-2: Waves of the Starless Sea" references the 1974 King Crimson song "Starless".
• "5-3: Ship of Fools" references the 1970 song by American psychedelic rock band the Doors.
• "5-4: Leviathan" references the 2004 album by American heavy metal band Mastodon.
• "6-1: Cry for the Weeper" references the 2005 song by American drone metal band Sunn O))).
• "6-2: Aesthetics of Hate" references the 2007 song by American heavy metal band Machine Head.
• "P-1: Soul Survivor" references the 1972 song by English blues rock band the Rolling Stones.
• "P-2: Wait of the World" references the 2013 song "The Wait of the World" by American avant-garde metal band Kayo Dot.