Languages with built-ins
HBL, 0.5: - - Try it online!
Fig, ~0.823: f - Try it online!
flax, 1: F - Try it online!
Japt, 1: c - Try it online!
Jelly, 1: F - Try it online!
Vyxal, 1: f - Try it online!
05AB1E, 1: ˜ - Try it online!
Pyt, 1: Ƒ - Try it online!
J, 1: , - Try it online!
Nekomata, 1: V - Try it online!
MathGolf, 1: ─ - Try it online!
Pip, 2: FA - Try it online!
Pyth, 2: .n - Try it online!
Attache, 4: Flat - Try it online!
Raku, 5: &flat - Try it online!
rSNBATWPL, 5: crush - Try it online!
R (almost), 6: unlist -
Try it online!
Clojure, 7: flatten - Try it online!
jq, 7: flatten - Try it online!
Factor, 7: flatten - Try it online!
Haskell + free, 7: retract - Try it online!
Kotlin, 7: flatten - Try it online!
Mathematica, 7: Flatten - Try it online!
Prolog (SWI), 7: flatten - Try it online!
Racket, 7: flatten - Try it online!
Ruby, 7: flatten - Try it online!
Feel free to add to this community wiki. The below is a template to copy the code and links into to add to the above list.
[<language>](<language URL>), <byte-count>: `<code>` - [Try it online!](<interpreter url>)
