Star Trek: 10 Classified Starfleet Secrets

In Star Trek: Discovery's third season, the ship received a number of tech upgrades from the future. One of these upgrades was a cloaking device, which surprised audiences because, for most of Starfleet's history, the organisation was banned from developing any sort of cloaking technology.

The ban was one of the requirements of The Treaty Of Algeron, which also reinforced and redefined the Romulan Neutral Zone, and greatly improved the Federation's relationship with the Romulan Empire. The treaty brought peace to the Alpha Quadrant, but delayed the advancement of cloaking technology for centuries.

Many in Starfleet were apparently rather displeased with this arrangement, as we learn in the Next Generation episode Pegasus that the organisation illegally developed and tested a secret cloaking ship, the USS Pegasus. The Pegasus was equipped with a phased cloaking device, a unique type of cloaking tech that shifted the vessel out of phase with normal space, rendering it not only invisible, but also presumably indestructible.

After an accident, the Pegasus was thought to be destroyed, but it was actually drifting through space in its cloaked state, before materialising later inside of a solid asteroid. In the episode, the Enterprise-D crew was ordered to retrieve or destroy the wreckage before the Romulans found it, but when Picard learned the truth of the phased cloaking device, he made the information public.