Thanks for the link, Frabby. I think this piece sums it up:
As a defining aspect, LosTech cannot be produced anymore under current technology levels, except occasionally by automated facilities which are invariably LosTech themselves. Often, the working principles of lostech items are not even understood anymore and the secret of their production is lost. This became a growing problem when more and more vital industry, including automated factories, collapsed from maintenance shortfalls, either because nobody knew how to properly maintain the mysterious machinery or simply for a lack of spare parts. In the dark days of LosTech, only scavenging and improvisation kept any advanced technology working at all.
There is a conspiracy theory that a certain organization actively advanced the decline of technology through sabotage and assassinations. I phrase it that way in order to avoid spoilers. I give some examples of lostech:
Fusion engines are essential part of BattleMechs. Those are in production, but by early 3000s production output is so small that perfectly working tanks and other armored fighting vehicles are stripped out of them for use in 'Mechs. Those vehicles get internal combustion engines as replacement which reduces their speed and/or other capabilities. Record Sheets: 3039 Unabridged has some examples, at least J. Edgars (ICE Variant) and Schrek Autocannon Carrier.
Double Heat Sink was made as early as in 2600s, if not before. It weighs 1 ton, takes 3 critical slots, and dissipates 2 points of heat per turn; in short weighs the same as Single Heat Sink but works at double efficiency. Those aren't available in year 3000 anymore.
Extended Range Large Laser weighs the same as Large Laser but has more range. Like with Double Heat Sink, all production facilities got nuked and by year 3000 there aren't any (publicly known) working examples anymore.
In 3020s begins technical recovery. Hanse Davion raids a library in world Halstead Station in Draconis Combine where 'Mechs stuff the books into 'Mech scale sacks to carry them off. Those books and information within are used to create most advanced and sophisticated university in the Inner Sphere, in Federated Suns of course. Gray Death Legion finds an electronic library (novel Price of Glory) of lost knowledge and distributes copies of it everywhere they go. Naturally copy makes it to Davion's new university. Even without access to such data, Lyran Commonwealth builds new factories to make new fusion powered combat vehicles (Savannah Master, Hunter, Drillson, Patton, Rommel) and new BattleMech designs (Hatchetman, Wolfhound). In addition Lyrans send their engineers to fix up 'Mech factories in Federated Suns and restore production of 100 ton Atlas 'Mech over there. Mad scientists of Capellan Confederation have their own research projects, which eventually lead to Triple-Strength Myomer and Beagle Active Probe. Between 3030-3049 Double Heat Sinks, ER lasers, and Pulse Lasers return to production alongside with other equipment that was, and new stuff comes along the way.