I’m not sure how much actual effort it takes to make sure Plasma keeps compiling for X11, but based on the wording from the blog post it sounds like they’re exerting pretty much as little effort as possible. I would say with the recent uptick in leading-edge distros moving to Wayland it’s only a matter of time before almost no one is left on X11, which will deprioritize it even further. Pulling the plug on X11 today is premature given how many people are still running it (SteamOS uses it by default, for example), but I think their ~2 year estimate sounds about right for letting off the gas and putting a hard stop on support.
I’m used to LanguageTool, and at a glance it seems like Harper covers way fewer rules than LanguageTool does. Not sure if this is actually noticeable in practice, but I run my own LanguageTool server and am not too picky about the performance, so I’m not in a rush to move until someone figures out a good way to compare them. LanguageTool’s rules are all open source at least, so it’s only a matter of time before Harper gets anything it might be missing.