Warning: This article contains spoilers from The Bad Batch season 3, episode 15, "The Cavalry Has Arrived."
Even though Project Necromancer began in the early years of the Empire, The Bad Batch series finale explains why the project is still being worked on decades later during The Mandalorian.
Project Necromancer was first mentioned in The Mandalorian Season 3, episode 7, "The Spies," as Moff Gideon met with the Shadow Council. Captain Gilad Pellaeon declared that Grand Admiral Thrawn's imminent return would give Commandant Brendol Hux time to complete Project Necromancer. When Gideon called for new leadership, Hux assured him that Project Necromancer would deliver on that.
The Bad Batch's third season shows that Project Necromancer began under the supervision of Dr. Royce Hemlock at Mount Tantiss, with Emperor Palpatine himself being deeply invested in the project. The goal of Project Necromancer was to successfully clone an individual's DNA along with a high level of midi-chlorians, also known as M-count. The issue is that M-count always degraded during the cloning process, which is why Omega was so valuable to Hemlock, as her blood was the only binder that prevented M-count from degrading during a transfer.
Despite these discoveries and the progress made under Hemlock's supervision, Kaminoan scientist Nala Se irrevocably sets back Project Necromancer by destroying the databanks and all research in the central lab on Mount Tantiss. Nala Se sets a thermal detonator to explode, and the explosion is triggered when former Vice Admiral Rampart shoots her. This causes the charged detonator to hit the floor and obliterate everything in the lab, including all information on Project Necromancer. It killed both Rampart and Nala Se in the process.
Hemlock and Dr. Emerie Karr were the only other ones who truly knew and understood all the research done so far on Project Necromancer. That knowledge dies with Hemlock, and Emerie abandons the Empire to help her clone brothers and sister. The death of Project Necromancer is further confirmed when Governor Wilhuff Tarkin arrives at the ruins of Mount Tantiss and learns that none of the research is salvageable. All funding for Project Necromancer and Hemlock's other research projects are now diverted to Project Stardust, the codename for the first Death Star.
It is deeply satisfying to know that Nala Se is the one to sabotage Project Necromancer. In The Clone Wars, her actions contributed to the death of clone trooper Fives and prevented the truth behind the inhibitor chips and Order 66 from being discovered. This helped Palpatine destroy the Jedi Order, transform the Republic into the Empire, and force almost all of the clones to kill the Jedi they fought alongside for years. The Empire later destroyed Nala Se's home on Kamino, Tipoca City, and Hemlock used Omega to blackmail Nala Se into working on Project Necromancer against her will.
Nala Se's actions in The Bad Batch series finale allows her to get revenge against Palpatine, Hemlock, and the Empire, all of whom underestimated and undervalued the Kaminoans. More importantly, it helps Omega and the rest of the clones remain free from the Empire. Without Project Necromancer, the Empire has no reason to pursue the clones anymore. Not only does Nala Se help the clones whose agency she compromised, but by choosing to do the right thing this time, she saves the galaxy for years to come.
The failure of Project Necromancer means that Palpatine is unable to be successfully cloned before or during the Galactic Civil War. Thanks to Nala Se, the Imperial Remnant led by Brandol Hux are still trying to unlock the secrets of Project Necromancer and are unable to resurrect Palpatine via cloning by the time of The Mandalorian Season 3. They may not even be close to knowing or achieving any of the results achieved at Mount Tantiss.
While Palpatine eventually returns in corporeal form with a high M-count as seen in The Rise of Skywalker, he's a rotting husk of his former self and can only exist while hooked up to machines. Draining Rey and Ben Solo of the combined life force of their Force dyad was the only thing that could bring him back to full strength, not any of the scientific research from Project Necromancer.
If Nala Se hadn't destroyed all the research on Project Necromancer, Palpatine likely would've returned and at full strength much sooner, making him potentially impossible to defeat and able to conquer the galaxy once and for all.