In the recent Diablo 4 PTR Patch Notes, Blizzard confirmed that Forgotten Soul will drop from additional sources in Season 4; including Whisper Caches and as a rare drop from any Elite enemy.
Diablo 4 PTR Patch 1.4.0 Patch Notes Fixed Damage Legendary Aspect Skills
Blizzard
Forgotten Souls can now be obtained from Whispers and are now a global rare drop from Elites.
Players will no longer need to grind Helltide in order to acquire Forgotten Souls.
