In this DungeonCraft episode, Professor Dungeon Master looks at the early West Marches/open world campaigns that Gygax, Arneson, and other early DMs ran.
He explains the differences between such open world campaigning and modern campaigning and describes how you can incorporate this campaign style into 5th edition Dungeons & Dragons.
One of the more fascinating parts of the video is the backstory on some of the first edition rules that contemporary players may find odd (like 1 gold piece equaling 1 experience point and 1 day in-game equaling 1 day IRL) and how these rules played into the West Marches campaigning style.
Here are the Professor's basic rules that he uses in running modern versions of this old-school campaign style:
Here is an excellent Questing Beast episode with a deeper dive into these open world gaming concepts: