Solo RPG has been there from the beginning:

Solo RPG has been there from the beginning:

"Although it has been possible for enthusiasts to play solo games of
DUNGEONS & DRAGONS by means of “Wilderness Adventures”,
there has been no uniform method of dungeon exploring, for the campaign
referee has heretofor been required to design dungeon levels.
Through the following series of tables (and considerable dice rolling)
it is now possible to adventure alone through endless series of dungeon
mazes! After a time I am certain that there will be some sameness
to this however, and for this reason a system of exchange of sealed
envelopes for special rooms and tricks/traps is urged. These envelopes
can come from any other player and contain monsters and treasure, a
whole complex of rooms (unfolded a bit at a time), ancient artifacts,
and so forth. All the envelope should say is for what level the contents
are for and for what location, i.e. a chamber, room, 20’ wide corridor,
etc. Now break out your copy of D & D, your dice, and plenty
of graph paper and have fun !"
(Gary Gygax, The Strategic Review, 1975)

you can google it and find his solo dungeon creation system