I heard about this the other day when a friend of mine told me that he had an e-mail from GitHub about this... unluckily for him, he was one of the people who's password was left in plain-text.
Thankfully mine wasn't.
Let this be a lesson for those who do not use different passwords for different services though. GitHub are a reputable service of course, and I trust them personally, however it is important to remember that if someone can access your password in plain-text, unless you are using a different password for different accounts on different services, they could then compromise all of your accounts just from having that one password. It is really important to stress this because I know people who have blatantly ignored practices such as using different passwords for different services, and I have then witnessed them eventually wish they hadn't have re-used the same password.
For those who use a lot of services and are unable to remember different passwords which are also "secure" (and by "secure" I am referring to having a lot of characters, a combination of upper/lower case characters, numbers without a sequence and special characters), I recommend a good password manager. There's many available and with some trial and error you should be able to find one that works for you; at-least this way you can enforce safe passwords on different services, and also not re-use them more than once.
As a final note, I recommend changing your password every few months at a minimum. Breaches happen all of the time, and some companies may hide it up for years before you find out the truth (as we've seen in the past). If you at-least reset your password every few months, you would be doing yourself a favor. Resetting it every month would be even safer in my opinion.
There's a service out there which does let you check if your password has been in a proper leak dump, while it won't be 100% reliable since it will be relying on dumps the developers are aware of, it is better than nothing and does work well. You can find it here:
Have I Been Pwned: Check if your email has been compromised in a data breach
Remember that this isn't to do with the GitHub situation, the passwords which were in plain-text were not leaked nor seen by anyone except a few GitHub staff who worked on identifying and solving the issue. All of this is simply as a reminder/guidance for those who do not already have good password security practices. If you want to be safe online, you have to keep yourself safe.