Re: How is this possible ? A Recent transaction suddenly becomes invalid
If they didn't know how to do an RBF-enabled double spend, they now have all the necessary information. Even if it's a great post, it will surely be misused. 
RBF isn't exactly some top secret information. 10 minutes and the ability to use Google and anyone who wants to try to scam people by double spending with RBF will know what to do.I still haven't upgraded my Electrum from 3.3.8 so the steps are a bit different on the old version.
Why? There's no good reason to use an outdated version. Even if you aren't interested in using Lightning, version 4 and up contains a couple of other nice new features and a whole ton of bug fixes.