The history of Dungeons & Dragons as a franchise is a bit confusing. Formerly called Advanced Dungeons & Dragons (AD&D), this is no longer the case. What happened? As ScreenRant explains, the transition happened with the help of Vecna.
Old school D&D is, well… difficult. There are a ton of rules, even more restrictions, and THAC0 was a thing. Long story short, when AD&D Second Edition came out, it was produced by TSR. TSR would change hands and eventually go bankrupt before being acquired by Wizards of the Coast, in 1997. WotC completely overhauled D&D to make it much friendlier to new players, and this included major rule changes, streamlining of the game, a lowering of the learning curve and changes in the game such as the addition of a wizard. (It’s a class that has innate spellcasting abilities, as opposed to a wizard who needed to study to learn magic.) Combat became much more tactical, most things could be settled with the roll of a d20, there were no limits on class, combinations of races, and characters could progress in prestige beyond level 20 (via DiceBreaker). However, they didn’t just erase everything before the new 3rd. Instead, they turned to Vecna.
Thanks to Vecna’s invasion of the city of Sigil, the shockwaves the fight has sent across the multiverse, and the Lady of Pain’s attempts to hold all of existence together, changes between AD&D and D&D were actually canon. All those rule changes? Class changes? This is what happened after the Lady of Sorrows put the world back together after Vecna’s attempt to smash it.