Cam is shot and probably rod,main bearings etc look like hell, Oil nowdays ain't good for flat tappet cams, you need to run a additive with zinc in it,Lucas makes a good one..
Improper cam break-in, lack of zinc in oil, poor quality metal in the cam and lifters, or an combination of the above. Given that it's dished that bad and worn through the base of the lifter, you really should tear into the engine, check bearings, etc. because there's been a whole lot of metal powder produced from that failure.
Either not enough oil getting to the lifter or oil that's not lubricating properly. These days I run only "racing oil" because it's exempt from having to pull out the zinc and other anti scuffing additives. I used to run diesel oil but it now also has had to remove the anti scuffing chemicals. There are almost no modern engines sold today that don't have roller type lifters, so we are the only ones suffering that type of damage due to flat tappet lifters.
It could also be from initial start up problems. Usually because the engine was not pre lubed properly or did not run at 2000+ rpm on initial start up.
2x on tearing the engine apart. I put a new cam/lifters in a running 427 a few years back. One cam lobe went flat (lifter looked just like yours). I changed the oil a couple times, but it was too late...ended up with 4 spun rod bearings.
A friend bought some lifters and made the comment that they only cost like $4.00 each..and they looked like that after ten minutes..Cleaned the cam up with a stone, bought quality lifters and has been running fine for years...
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