I did point this out, but I expect you failed to see it or accept it. By not using third-party repositories which have no guarantee whatsoever, you limit your exposure and therefore less likely to have a broken system. Had you done this, your system would not have been affected because the only problem encountered because of it was linked to KDE. Which was also pointed out isn’t in the official repositories.
The more third-party repositories you start to add to your system and install packages are, then you are exposing yourself to the possibility of instability.
Unfortunately bugs occur, and not necessarily through the fault of RHEL or even the third-party repositories. RHEL is designed as a stable OS and has older packages mainly for that reason - it’s not like Fedora for example or Debian SID/Unstable where more bugs/problems/instability are likely to occur. I have used Debian SID/Unstable and yes from one day to the next something did break. I upgraded Libreoffice, and it stopped working, wouldn’t launch. Then a few days later it was working again when I next applied updates to the system.
Let me also point out, if you had been using RHEL with a subscription and then complained to them that your KDE system broke when you upgrade to 8.6, what do you think their response would have been? It would be: “We don’t have KDE in our repositories and therefore we do not support it. That sounds like something you have added to your system and therefore that problem lies with the third-party repositories that you added, or the person who compiled and installed it on that system and you should be speaking to them for a resolution to that problem.”
And sure, you could attempt to convince them it was because of a bug in the kernel, and as I have said, I have no problem with accepting such a scenario because bugs do happen, but with RHEL support you won’t get anywhere with that. Mostly because they are likely to say, does your system work perfectly fine with Gnome (of which is the desktop supplied by RHEL)? Your answer to this of course is going to be yes, which means the only thing affected by it is KDE which is not supplied by them. At which point, it’s not Red Hat’s fault because Red Hat can 100% prove that their system works perfectly fine with everything that comes in their repositories by default. Even if they were to go through a court of law, they would win because a third-party repository has been used and they only have to make sure that the kernel works perfectly fine with their system.
You can also quite easily build the kernel yourself with the option you needed to make KDE work, or grab a kernel from elrepo or other repositories if they have that open enabled as well. I mean, as you are using one third-party repo for installing KDE, then it’s not much trouble to add another one and grab a kernel that would help get the desktop of your choice working properly. And then everyone is happy.
You cannot 100% blame RHEL for this, when you were the one who added third party repositories to your system. Partial blame lies with yourself, whether you want to accept that or not. Just like the blame lay with me for using Debian Unstable when one day Libreoffice stopped working, and then a few days later it started again. It was my choice to use it, therefore I can only blame myself for using it in the first place. Had I stuck with Debian Stable, then I wouldn’t have had any problems whatsoever. Just the same as if you had done the same with Rocky and stuck with the official repositories.