Tech-Thralls, also known as Adsecularis, are the poor buggers considered even more expendable, and thus ranked even lower, than other types of combat servitors. They are used when a need arises for expendable servants or troops. They are created through basic augmetics and cranial surgery and often captured prisoners or criminals are used for the process.
The ultimate fate of most Tech-Thralls is to end their lives recycled into Servitor components and their implants harvested for further use. On the bright side, even though regulated as cannon fodder, the Tech-Thralls are equipped on par with your typical Imperial Guardsmen's Flashlights and T-shirts, being protected by same combat armor and armed with a variety of light weapons such as Shotguns, Chainblades, Las-Locks and Mitralocks. It really does show that despite their incompetence, the Mechanicus is still the most advanced and stable polity in the entirety of the Imperium where their literal cannon fodder can put up a fight against the Emperor's finest. It is further infuriating that despite having all the goodies, their secrecy, internecine rivalries, and unreliability have hampered the Imperium on some occasions despite having technology surpassing the Tau in many areas and even rivaling some Eldar equivalents.
Tech-thralls were specialized troops of the Legio Cybernetica created by the Mechanicum through basic augmetics and cranial surgery as the need for them arose, especially during the massive Imperial expansion during the Great Crusade and later the interstellar binge party known as the Horus Heresy. They were not Servitors in the conventional sense as Servitors used often formerly human biological matter as a component system, but rather could be considered cyborgs as consciousness and memory was not extinguished before they were reprocessed into their new forms, merely mutilated and overridden.
The conversion process commonly awaited the laity of a Forge World's population should it prove sub-standard or commit some transgression against its masters, for few other criminal punishments existed in the Omnissiah's domains. Tech-thralls served many purposes, but principally that of a flexible pool of expendable labor on the Forge Worlds. But in times of war, this same fate befell many to create a quickly-produced and ultimately expendable military resource known as the Adsecularis. In these cases, the organic human bodies of the Tech-thralls, already altered to lift heavy loads, function in hostile environments and obey the orders of their masters until death. Tech-thralls were fitted with 'basic' weapons systems and bodily protection. On the battlefield they served as expendable troops, unskilled but relentless and indefatigable; an army of the alive-yet-dead, puppeteered by the Mechanicum Magi who commanded them.
It is unknown whether they are still in use by the 41st millennium, although the chances are quite high just by looking at the amount of slave labor going on in the Mechanicus.