So, is it mandatory, or not ?
Doesn't matter. The game's still not DRM-free by any margin and you'd need the Steam version to avoid installing the really fucking nasty malware* it comes wrapped in, regardless of what the patches may disable. Remember that SecuROM is near impossible to remove without a format, which makes some of those earlier, really bloody nasty incarnations the kind of thing you don't want to let near your system, no matter what.
*Yes, it is malware.
... Doesn't Diablo II have Battle.net DRM?
My disc copy certainly doesn't. Never once in my life installed Battle.net.