Related
The nextDungeons & Dragonsanthology will include several adventures meant for solo play.
No further elaboration was provided on how these adventures are made with solo players in mind.
These rules were later fleshed out inTasha’s Cauldron of Everythingbut have not been expanded upon since.
Specifying that an adventure is suitable for solo play is a big departure forDungeons & Dragons.
Source:D&D Beyond