This bug means that when somebody uninstalls Signal (or an iPhone disables it for lack of use), people sending messages never learn that the message didn't go through.

As a result:

  "I just missed out on placing an offer on a house because of this issue."

  "Today I ended up uninstalling Signal after an experience where this problem caused me to almost lose two of my friends."

  "I just had the experience of being very worried about a friend's well-being because he stopped responding to my messages."

And so forth. It makes Signal a very risky thing to use, not because the encryption is bad, but because the UX is. It seems easy to fix, but Signal never seems to care.