This flag was introduced in PR #784, after a commit in PR #739 unconditionally stripped ANSI codes from stdin. PR #739 was made in response to issue #689; in order to prevent the display from being garbled by incorrect ANSI code handling, we opted to simply strip ANSI codes from the input entirely. The implementation in PR #784 (and my own draft implementation in PR #782) simply undo that stripping behavior without addressing the underlying issues from #689, resulting in strange outputs in certain cases. Below is a minimal example which results in garbled output on my machine in Kitty, Alacritty and st.

#!/bin/sh

green=$(tput setaf 2)
reset=$(tput sgr0)

{
  echo "Option 1"
  echo "Option 2"
  echo "${green}Option 3${reset}"
} | gum filter --no-strip-ansi

Here are recordings for each tested terminal. Note that Kitty was a lot worse than the others; interestingly, when I tried using asciinema to record Kitty's output, the recording showed the same 2m result as the other terminals; to show what it actually looked like on-screen I had to use a screen recording.

1. Kitty
2. Alacritty
3. st