My 2 penneth for what it's worth, The forum is exactly that, a forum and not a shop. It is not easy to ensure that posts people have made in the past are still valid, reflect the latest 'best practice' or even still work. Even for the original poster who may not be using the code anymore. Any source code provided for free therefore cannot have an absolute guarantee that it will still work. From what I have seen, most posters when alerted to the fact there is a problem with something they have posted in the past will try to resolve the issue wherever possible, if they are still active on the forum.
Even so, if you are going to rely on a library or source code provided for free, and in the case of source code, you don't have the skill to or interest in understanding how it works and maintaining it yourself, then it would make sense to contact the author and ask if it will still be supported if a problem arises.
Source code for sale is a slightly different case to my mind. Charging someone for anything implies some kind of warranty, which may be limited to the fact that it worked with the version of Java / Android OS/ IOS that it was created on.
It would make sense to me that before you part with money for something, you make sure that the vendor is still active on the forum and check that any reported issues have been resolved satisfactorily. This is something I do regularly when purchasing software from any website, it's all to easy to think that something looks good and it's only x pounds, and make an impulse buy, only to find out that it is out of date, or not working on the latest OS or worse it is just an out and out scam, not that I am suggesting anyone on the forum would do that.
Just be diligent, and if you are unsure, contact the vendor and ask. If you can't contact them, avoid it.
This is all easy in hindsight and if you've bought something and can't get delivery it leaves a bad taste in your mouth. As a last resort, if you paid using paypal or similar, you may be able to ask them to help resolve the issue or get your money back.