GM issued a directive that guides actions under warranty by build date (truck build date). But that is just to give the dealer guidance on the required work if a failure occurs.
The actual cutoff date for bad lifters being installed in engines is March 5, 2021. Engines assembled before that date may have faulty lifters. The actual time delay between an engine being completed and the truck it is destined for being "completed" can be quite long (but I think normally is within a few weeks, hence the 3/31 cutoff in GM's warranty work directive).
You can see the actual engine build date by looking up under your truck at the back of the engine, on the drivers side. You will see a white sticker with a letter code (what plant it was assembled in), a 1 or 2 next (first or second shift), then a Julian date of the year and day it was assembled, the rest is the sequence code.
All of the above was posted on another forum by a guy who works at a GM engine assembly plant. He explained the faulty lifters suffer from the failure of an internal spring that locks the lifter so that cylinder is in fire mode. When the lifter pin lock spring breaks that lifter's valve will not open and you get a solid misfire in that cylinder.
He also explained that the suspect engines are also subject to faulty valve springs being installed (also due to improper heat treating), which explains the bent push rods on some engines (valve spring breaks, valve drops down into the cylinder where it it impacted by the piston and sent back up and eventually to a pushrod that is lifted by the cam and lifter in that cylinder on the opposite cycle stroke for that valve, bang, push rod does its job and bends minimizing other damage. Some speculate that this sequence of events can also cause a DFM lifter failure (due to the force of the push rod impact), I have no idea if that is correct or not, but it does sound plausible and would explain the "found: bent push rod and collapsed DFM lifter" report.
