I've done a lot of solo game with my sons using DnD or No Thank You Evil with either me or a son as a DM and it is always fun.
With my regular adult group, there are rare instances where only one person can make it. As the DM, if it makes sense based on where the party ended up last game, I will run a solo.
In one instance it was a heist game, where the person had to infiltrate an inner circle of elites, get invited to an open house with art (including the macguffin) on display, and latter steal the item.
In another, I ran the player through Forge of Fury in Tales from the Yawning Portal with a character that was much higher level than recommended for that adventure. It was challenging as a solo and was a surprisingly fun session for both of us.
I still prefer group play, but I don't shy away from solo (one player and one DM) games.
As for true solo play - just one player using solo-play mechanics instead of a DM...I've never been into that. Even solo card and board games don't interest me. I would rather read a book, watch a movie, or play a video game.