I read on investopedia.com:
If it's a Chapter 11 bankruptcy, common stock shares will become practically worthless and will stop paying dividends. The stock may be delisted on the major stock exchanges, and a Q may be added to the stock symbol to indicate that the company has filed for bankruptcy.
Why is a ‘Q’ added to the ticker symbol of a bankrupt company?
Q is for "Caution". The letter itself denotes that the ticker is for a company in bankruptcy proceedings. Which you knew already since you said so in the question.
There are many letters used for different situations, Investopedia lists some of them. Trying to guess the meaning just by looking at the letter is pointless. They may differ between exchanges (e.g.: NASDAQ no longer uses the Q, but NYSE does). The historical context on how the letters were chosen is irrelevant for making financial decisions.
According to Reddit, the Q doesn't stand for anything. Quotes below.
From SirGlass:
I don't think it means anything , Some of the symbols relate to the letter , A means class A shares, B means class b shares , W means warrant, R means rights issue , F foreign shares; but then lots just were assigned a random letter. So I do not think the Q means anything they just had to pick a random un-used letter because B was already taken. https://www.investopedia.com/ask/answers/06/nasdaqfifthletter.asp
From royhenderson771:
It doesn't stand for anything. It simply acts as a way for investors to know the company filed for bankrupty. From the SEC: “Q” is for Caution