With the upcoming 1.11 patch Paradox will be retiring some old Linux and Mac platforms that aren't maintained by their own developers anymore, but more importantly, switching to DirectX 11.
That's right, the next HoI4 release will join Stellaris and the other more recent PDS games by adding DirectX 11 support. DirectX 9 will still be the default for now, but you will be able to select another renderer in the launcher settings.
While there shouldn't be any visual difference between DX9 and DX11, and OpenGL for that matter, developers have been working on making sure the experience will be the same. The game should load a bit faster, however, especially if you use a lot of mods that bring extra textures and models. Working remotely for more than a year, Paradox also noticed fewer issues playing with remote desktops and the like, which might not be a big deal for most of you but was quite appreciable for the devs during development.
Finally, it did help in tweaking and fixing the new railways and trains graphics, as most graphics debugging tools have dropped support for DX9.
Why do they make these decisions you may wonder? It can all be boiled down to two main reasons, as stated above, maintaining old tech comes with a cost that Paradox would rather spend on improving the game for 99% of the users, and sometimes it blocks the adoption of new tech that would make the programmers' job easier and more efficient.
For those of you still using Windows 7, don't worry, that particular OS is still not the chopping block, which can't be said for other OS, such as Ubuntu and macOS.