YouTuber ReznoR has made a few videos upscaling footage of old games, the latest of which shows off the opening 16 minutes of Diablo 2 at 4K resolution and a framerate of 60fps. As he explains, "I used Deep Learning Neural Network AI [also known as Machine Learning (ML)] and Advanced Motion Interpolation to upscale, enhance and smooth the original Diablo 2 gameplay." Keeping the original aspect ratio means that it still looks like the Diablo 2 I remember, just much smoother.
The sound's been tweaked as well: "As a bonus the EAX Environmental Effects and 3D audio known from old Creative Sound Blaster Audigy/X-Fi cards are enabled in this demo as well as Head Related Transfer Function (HRTF). Headphones are recommended for best results!"
To reiterate, this is just upscaled footage that was captured playing the game at ye olde 25fps and 720p. Machine learning filled-in the missing frames, which is why those animations look so nice. Make sure you've got the video's settings turned up to max for the full effect, of course.
While there's a rumor Blizzard are working on a remastered version of Diablo 2 this year, it's still just a rumor at this stage.