Walkthroughs for custom maps.

As for the "pure" Heroes, with the HD mod and the HD+ option turned off, we have exactly the original gameplay with all the original bugs.

It can be clarified that it includes all the original non-critical bugs. The HD mod also addresses critical bugs (i.e., those that cause the game to crash) even when HD+ is turned off.

Added 10 minutes later

Here, it's less about principle and more about a stubborn mindset – the idea that only the "pure" Heroes are acceptable. This doesn't lend itself to rational explanation; it requires a psychiatrist.

The fact that people have such a personal (almost religious) attachment to Heroes 3 can't be dismissed as a psychiatric issue. I understand you perfectly.
And using the HD mod certainly has its drawbacks (along with its advantages, of course).
Firstly, the atmosphere and quality of the game are undoubtedly diminished when using it (the music at the start of battles, the wrong spellbook, "foreign" interface elements, etc.). And while in older versions (up to version 4), almost everything that didn't appeal to you could be turned off, now almost nothing can be turned off.
Secondly, the HD mod, while fixing bugs in the original game, introduces its own bugs, although they rarely affect the game itself.
Thirdly, the HD mod is essentially undocumented. There is no description of the HD+ option – how it specifically changes the original game, and there is no documentation in general. There is a changelog, but it doesn't describe everything, and not always accurately.
The author of the HD mod explicitly states that he does what interests him (for example, documenting or fixing many bugs in the HD mod doesn't interest him), and others are free to use the HD mod or not.

Nasos, if you're interested, I can help you find a version and settings for the HD mod so that everything looks the same as before, but you can still use hotkeys for quick army management, and so that the original game fixes work.
At the same time, I can give you a DLL module to disable the new backpack view and the new battle results window (this can't be done simply by adjusting the HD mod settings).