There are two separate branches of QTTabBar. This is the most recently updated (and stable/feature-rich); and this is the one by the original developer.
That one is certainly not holding this one back in any way. But the developer (here) seems to have disappeared/not come here for at least a year. He may turn up at some point.
My guess is that he is simply busy with other projects/work. I suspect that he was not getting many (if any) donations. Plus, sooner or later anybody has got to get frustrated by ignorant fools who can't read instructions, guides or manuals that took many hours to prepare- but they wanna get onto forums and bitch. They typically ask simple questions (often barely intelligible), demanding answers that they would already know if they had bothered to read any of the documentation.
It is unfortunate that the two branches have exactly the same name and that there isn't an announcement (preferably on both sites) concerning the existence of two separate (and incompatible) different branches of the software. But I believe the developer here feels like he doesn't need to acknowledge or mention the other branch because his is the original. And the developer for the other branch totally disappeared after a bunch of delays of a promised major update. And all the articles about this software that I've seen do not mention that there are two branches; which is the latest version; or anything like that…