Grimoire Cards in Destiny, as the Destiny Wiki explains, are "awards that players earn when they perform certain actions," like killing a particular guy or finding a new location. They can improve experience gains or drops, but their primary purpose is to tell the story behind the game by fleshing out the lore related to characters, weapons and locations. Somewhat maddeningly for players, you can only actually read the cards outside of the game, either on Bungie's website or the official app, which naturally kept a lot of players from bothering.
So for Destiny 2, the plan is different: As world lead Steve Cotton told Forbes, Grimoire Cards are being ditched in favor of in-game storytelling. "The answer to that question [about Grimoire Cards returning] is 'no,' and the reason it's 'no' is because we want to put the lore in the game," he said. "We want people to be able to find the lore. All the story is told through the Adventures, it's told through the characters in the world, it's told through the campaign, and it's told through scannables you find throughout the world."
No doubt some players won't care about all that narrative guffola, but for those who do enjoy the lore side of Destiny this is definitely a welcome change. Shooters don't necessarily need a story to work, but if a studio is going to bother putting one in, then it shouldn't make players work more than they have to, within the context of the game, to get to it. Imagine, for instance, if you had to log into the Irrational website to listen to the audio recordings in BioShock—that would not fly.
Destiny 2 on the PC doesn't yet have a release date, but it will be sometime after September 8, when the console versions are set to come out. If you're just getting caught up, we have some hands-on coverage from last week's big reveal event, and a rundown of everything we know about the game right here.