I’ve been corresponding with SmallCubed about this. The official word is that the old plugins work fine with 10.13 and earlier, and you can buy a MailSuite license that will work with the old plugins until MailSuite is out of beta. MailSuite beta is also supposed to be good with 10.13 and earlier, but I’m guessing that’s not quite 100% as they also mention the old software. 10.14 is the sticking point and I have no details on that—but an email mentioned that they were moving (home or office, I’m not sure) and that’s also been causing disruption.
I’ve corresponded with Scott Morrison for over a decade; he’s got a solid track record and it’s very unlike him to miss targets like this. I’m generally confident that the problem will be solved asap—but that said, this is one of two or three key reasons I’m still running High Sierra here.