I began looking into solo pen and paper RPGing a few weeks ago. This is a placeholder for if/when I continue to look into it.
There is an entire subgenre of solo RPG, journaling RPGs, which are popular on Itch.io. Thousand Year Old Vampire comes up a lot, but two other popular titles are The Wretched and Alone Among the Stars, both of which have inspired a large community of works.
On the non-journaling side, a popular choice is Ironsworn, which is available as a free PDF or a paid physical book.
Some solo their favorite RPGs using oracles and various story-telling devices, including Rory Story Cubes and card decks. The Motif Story Engine has my interest.
I have dabbled a little in journaling and in some not-quite-RPG solo games that rely heavily on tables. I’m somewhat amenable to the journaling but I’m very lukewarm on P&P that requires a lot of table lookup. I look at it this way: solo card games present all the information I need on the card. Instead of having to consult a table, the table is printed on cards and I always have the one I need at my fingertips. Since I prefer cards anyway table lookup just feels like busywork.