The "Lost Legends" inserted into these packs are said to come from a cache of Legends cases found in a closing warehouse in "the middle-of-nowhere in Washington state." WotC employees opened the packs, and certain cards found in those packs will be inserted into the Dominaria United Collector Boosters. These cards will replace a common foil in 3% of Collector Boosters for this set, and all rarity types within the Legends set will be represented in these cards.
The wonky collation of the 1994 Legends set is the stuff of legend in itself, which is why some cards will not be making an appearance in the "Lost Legends" slot. There was a massive collation error that occurred with Legends that played out as the cards were distributed and packs were opened back in 1994. Certain uncommon rarity cards were deemed "less random than they should be" by WotC. This was due to Cartamundi improperly loading the card sheets into the sorting and packaging machines in their factory. The error created two different lists of uncommons available to be pulled out of different pools of sealed product. The two different uncommon lists were dubbed by WotC as "List A" and "List B".
The collation problem with Legends was so bad that pre-Internet WotC had to put out an ad in issue #2 of The Duelist Magazine that offered a Legends Exchange Program. The program allowed players to trade in List A uncommons for List B uncommons, and vice versa; limiting the trade-in to 100 cards per person with no more than two of the same card able to be redeemed by a single customer. The Legends Exchange Program ran till August 31, 1994, and could speculatively be considered a possible culprit of the Legends booster product becoming "lost" in the first place.
Early WotC would've likely needed to keep extra Legends product from each List in close proximity to the home office to fullfill elements of the redemption process if enough cards came in. As evident from the list of cards not included in the Dominaria United Collector Boosters, the cases of boosters they found in the closing warehouse were mostly the List B grouping of sealed product. By the time the redemption program ended, WotC was knee deep into launching The Dark set, and it is possible that the product kept around for redemption was stored away in some warehouse and lost over time, akin to how the U.S. government shelved and lost the Ark of the Covenant at the end of Raiders of the Lost Ark.
A good percentage of the most desirable cards from Legends will be included in new Collector Boosters, like Moat, Tabernacle of Pendrall Vale, and The Abyss. Some desirable cards not included are Sylvan Library, Mana Drain, and Land Tax (all List A cards). Additionally, racially inappropriate Magic cards found in Legends will not be included in the Collector Booster packs (see "Racist 'Magic: The Gathering' Cards").
For more information on future releases of Magic products, check out ICv2's 2022 calendar (see "‘Magic: The Gathering’ Release Calendar For 2022/2023").