I have remembered that's was possible by using ECsc button, but don't remember what should to do with game before...

P.s. i'm not sure that a had played with it in original Diablo or some mod or Hellfire...

I don't think it's possible to get Diablo's drop in single player, at least not without some sort of mod.
In multi-player you can get his drop if you are with two people. One kills Diablo and is then forced out of the game by watching the ending. But if the other player was not on the same level when Diablo died, they continue playing and can get whatever Diablo left behind.

In my opinion it's not worth any effort though. He rarely drops anything for me.

In SP simply save while Diablo is bleeding, then load the game, now you can pick up the item and then save again so that the item will be in you inv for the next game

I never knew you could do that. Very interesting!

It's possible in multiplayer even playing solo I remember but how... because i had played long time ago Diablo and Hellfire original and also installed lots of different mod.

If you Alt+Tab during the ending movie, then Alt+Tab back into the game you will skip the ending movie. You will not be able to collect the loot diablo drops however.

You can use Diablo Saver to prevent the ending movie all together. A guide and safe download link can be found here:

https://d1legit.com/2019/04/06/diablo-1-diablo-saver-character-back-up-scan-others-for-dupes-hacks-and-more/

Diablo Saver is not legit.

If you use a damage over time spell (or some other spell and then use telekinesis you can pick a item up during the death animation. (I have not tried it but i think you could do that before)