I am sorry if I sounded somehow accusatory or denigrating. It wasn't my intent and honestly I don't feel that I have actually said anything that could make it look in that way. So please, do calm down and let's have a mature conversation if you want to provide help, or just avoid to reply.
I am a plugin developer myself, and trust me I would never come in a support forum to argue and accuse on the basis of nothing. I just noticed that nobody else reported this issue and wanted to report the bug. And as looking around I became concerned about the latency of the devs, I reached out to see if someone would actually be there to listen and debug this issue or if I'd better consider to switch to another plugin asap.
And by the way, as your inflammatory reply-style sounded familiar, I double checked the wp forum and you jumped at the throat of another guy on a review which was just saying that he had problems with the plugin (maybe his fault, but doesn't change the point) and nobody helped him because the forum and plugin are abandoned. And yes, there might be only one saying that it didn't work with wp 4.9.2 but there are so many other problems reported that didn't get a single answer from the devs, and the guy has all the rights to give 1 star review if he had a problem and nobody even replied to him.
I honestly don't know what pushes you to react in this way, especially if you aren't involved with the devs, but I would suggest to reconsider your reactions and try to breathe before replying. In particular considering that you are probably one of the few persons replying to support requests in the last months, and despite I too used to love this plugin, I am afraid it looks like it went down the drain, and maybe your efforts to save its reputations are not justified anymore and you're just lucky that you didn't stumble in a bug yet. (though 7 months without an update and active install dropped to 30k should ring an alarm to you too)
In any case, the only thing I said was reporting my problem and my suggestions based on what I know about it. If I already knew for sure what is the cause, I wouldn't be here asking, would I? So I definitely wasn't meant to come here and make accusations about a problem for which I don't know the cause. I actually came here to ask if someone knew the cause or had the same problem.
the only thing I know is that it was working before updating wp and now it doesn't.
and the problem is not related with paypal (at least it doesn't seem to, or not entirely) because it happens all within wp/s2members. I put the payment ID to generate the registration link from wp backend, it creates the link but when you click it says it's already expired.
and the payments are correctly created in paypal, but it seems that then wp/s2members doesn't send the emails. (paypal confirmation arrived correctly). And even in the case that it was because of a change in paypal API, it's still on the shoulders of the developers to quickly look into it and fix it, therefore it's still necessary for people to come here and report these problems. I always help people who says that have problems with my plugin, even if it's ultimately their fault. That's part of providing support.
after that said, if you have any suggestion about these issues I will appreciate your help, but I am mostly here to understand if any of the developers would ever look into this issue and to know if they are planning to release an update anytime soon, because otherwise even if this problem is caused by something else, I am afraid I'd better start the work to switch to another plugin asap (and definitely not happy to do so).
I will be pleased to hear a positive reply from a dev here, and it'd be also good for anyone else coming in the future and having the same scary feeling that the plugin is abandoned. No accusations and nothing personal. I just think it's a legitimate thought given the situation. And I am actually a bit surprised that I couldn't find any article talking about the fact that such a famous plugin seems to be abandoned...