From Halopedia, the Halo wiki
EXO/TSCS is an armour attachment that is compatible with MJOLNIR Mark V. EXO/TSCS is well-suited for exo-atmospheric engagements, and features an on-board Tactical Signals Intelligence Collection System.
The chest piece consists predominantly of a dark grey metal collar encircling the user's neck, set upon a thicker armored base that connects to existing the Mjolnir Mark V chest piece upon which it is layered. The two sides of the collar are cut off beneath the user's chin, and therefore do not meet to create full circle. Instead, the armored base on each side extends downward along the user's chest, forming two parallel strips which are joined at the sternum by a small connecting segment.
EXO/TSCS first appeared in promotional material for Halo: Reach's Defiant Map Pack in 2011, where it was worn by a player in three screenshots of the map Condemned. In addition to the EXO/TSCS, the player also wore the unreleased AKIS helmet with the GRD and CBRN attachments.
A lack of official communication about or acknowledgement of the armor from Bungie or 343 Industries led to a great deal of speculation about how it might be released, with some even suggesting that it could already be unlocked through some esoteric method. Eventually, these rumours were proven false when, on February 11th, 2012, 343 Industries employee David Ellis confirmed that 343 had intended to release it alongside the Anniversary Map Pack, but that it had been cut due to time and resource constraints. Ultimately, neither the EXO/TSCS nor the AKIS helmet chest piece were ever released for Reach on the Xbox 360.
The Spartan dolls Easter egg in the Anniversary Map Pack features dolls of Spartans wearing the same armor configuration as the player in the Defiant Map Pack screenshots, including the EXO/TSCS chest piece. This was a deliberate acknowledgement of the armor on 343 Industries' part. This doll object was later made available as a placeable object in Halo 5: Guardians' Forge mode, in which it is known as the "5th Spartan" doll.
In 2019, it was announced that Halo: Reach would be added to Halo: The Master Chief Collection, and that The Master Chief Collection would be ported to PC, with Reach being the first of the games in the collection to release on the platform. This brought with it renewed interest in the unreleased armor, with many requesting that it be included in the game's re-release. 343 Industries would officially address these requests in a development update in July of that year, saying that they "[were] currently investigating it but [could not] commit to it being in the game at [that] time" and also noting that they were interested in expanding the pool of customization items in each game, potentially even including new armors.
Later the same year, a modder by the name of Gamecheat13 discovered customisation renders for two cut armour sets in the game files of the second Halo Insider flight (dubbed the "FireFlight") of Halo: The Master Chief Collection's PC port of Halo: Reach, and also unearthed the internal codenames for both sets:
Eventually, in September 2020, it was confirmed in an MCC Development Update that these two armour sets would be making their way into Halo: The Master Chief Collection as part of Season 4: Reclaimer. This update also revealed the final name for the chest piece: EXO/TSCS. However, 343 encountered issues during the development process, and the armor did not yet meet their quality standards as of Season 4's release. Consequently, it was pushed back to an unspecified later season.
Defiant Map Pack promotional images
Halo: The Master Chief Collection datamined models
Halo: Reach MCC screenshots
List of apperances