I believe Tesla made 3 known improvements that likely had an effect. But still other failure modes exists. Tesla deploys revision changes mid year so probably can't just go by year alone. Nor do they publish a table on when these revisions are deployed.
1. Fuse Cover moved to Bottom (2016+?)
Fuse Cover on top side of battery gets attacked from windshield water runoff. Salted road regions and cars parked outside in high rain fall regions see much failure from this. Fuse cover on model X (released in 2015) moved to facing the ground so gravity + sitting water don't just eat at the seal / rust the metal cover. Model S parts catalog shows >= April 2016 manufactured cars likely have similar bottom facing fuse cover. However, it isn't so clear from Tesla repair directions which battery version got this update. Maybe one of the pack revisions indicate this change. I don't know.
Should be easy to see if have bottom facing fuse cover. Ground facing just behind the part+serial number sticker.
2. Front hump silicone valve air equalizer removed (2020-21+?)
I have seen pictures of newer packs (2020 or 2021) without this valve. This valve is likely the failure on
@mr_hyde 2015 pack as moist air (and/or standing water from windshield runoff) entered when valve opened to equalize air pressure. Moisture condensation inside the pack than attacks frontal pack electronics boards and other areas.
3. AC drain moved forward (2014+?)
AC drain moved from back of the frontal battery hump to front of the hump. Back of the hump is like a moisture trap and attacks the hump seam sealants. Front of the hump still dumps AC drain water near the frontal seam and with wind blowing backwards while the car is moving forward. It still attacks the seam metal and sealant on front of the hump but probably a lot less than the rear hump drain dump. A complete remedy should router the drain to clear exit point below the car and doesn't blow back up into the car.
Those with packs from after mid 2016 might fair better in moisture + salted road regions with #1 improvements. But of course #2 issue is still there as well as other failure modes.
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I think there are other improvements but its uncertain they had much of an impact
One prominent "improvement" could just be social media PR as the Tesla hired battery consultant (Canadian professor with battery testing lab) did many talks. This professor pioneered rapid testing of battery cell chemistry longevity. The theory is this allows faster chemistry improvement trials. This naturally gets picked up by tech social media youtubers which then lead people to believe rapid improvements like silicon chips Moore's law.
I have seen 2 Model 3 pack repair efforts. Both were shorted cells. Since rechargeable battery cell shorts over time+use is unavoidable. It isn't clear if this failure mode has been improved. Furthermore, this failure mode can occur with micron scale manufacturing defect.