The first game is playable but it’s only available on consoles, I haven’t played it myself but I heard it’s a good experience.

Sadly it won’t help you understand Destiny 2, as Bungie have completely butchered the game, removing key campaign-related content, entire playable areas (planets) of the game, and overall making the new player experience super confusing and unwelcoming.

The satisfying gunplay is unmatched but you have to consider if it’s enough to get into it