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!

Pip, 2: FA - Try it online!

Pyth, 2: .n - Try it online!

Haskell + hgl, 3: rtc

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!

Arturo, 7: flatten - Try it

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>)