Mass Effect Legendary Edition will include over 40 DLC from across the series, but that's still not all of the original DLC packs. Since Mass Effect 3's remaster is skipping the multiplayer mode, DLC related to that is being left out. And so is Pinnacle Station, an add-on for the first Mass Effect.
Pinnacle Station was originally developed by a third-party studio called Demiurge. It added a training facility full of holographic combat scenarios, and was generally considered pretty skippable. It was left out of the PS3 version of Mass Effect because the source code was lost, but when remaking the game for the Legendary Edition, BioWare apparently reached out to everyone they could to track it down.
When a backup of the code was finally unearthed, it turned out to be corrupted. While it would have been possible to reconstruct Pinnacle Station, it was decided that the effort wouldn't be worth it.
"It would basically take us another full six months just to do this with most of the team we've got," game director Mac Walters told Game Informer. "I wish we could do it. Honestly, just because this is meant to be everything that the team ever created, brought together again—all the singleplayer content. And so, leaving it all on the cutting-room floor, it was heartbreaking."
It's a shame, but Pinnacle Station was extremely forgettable. At least Mass Effect 3's fan-favorite Citadel expansion will definitely be part of the Legendary Edition.
This sort of thing is more common than you'd think. One of the most notable near-losses is Diablo 2, as Max Schaefer revealed at ExileCon 2019. "Not just our code, but all of our assets. Irrevocably, fatally corrupted," he said. Though they were able to reconstruct a lot of it, the assets that remain missing would make the rumored Diablo 2 remake harder to pull off.