You have a source for that? Every reloading resource I’ve ever seen instructs you to examine your brass and discard casings that are visibly damaged as a matter of course precisely because brass IS deformed to some extent with every firing and every reload, and WILL eventually fail.
The physics of shell casings makes this obvious, before firing the casing is gripping the round, and is not sealed to the barrel. During firing the casing releases the round and seals against the barrel, which requires it to flex under the stress. After firing the seal to the barrel is broken and the casing is extracted, this is more stress. Headspacing issues, rough handling, overloaded rounds, etc will all just accelerate this process. This might lead to an indefinite lifespan if the casing were designed for it and if it were a gradual process, but neither of these is the case (heh). Casings are designed for reliability on first firing, not reloading, and this process is in reality extremely energetic and fast, which means the stresses are significant. Directions for reloading necked (rifle) cartridges instruct you to trim the casing for length with each reloading. You’re losing brass material, this is obviously going to eventually lead to the casing breaking.
Anecdotally the lifespan of a casing seems to be somewhere between 5 (high-powered rifles) and 50+ (pistol rounds, particularly if lightly loaded and carefully reloaded). Anecdotes retrieved from forum posts found by searching for, “how many times can I reload a firearm casing”. This article: http://www.massreloading.com/longevity.html presents a simple longevity test performed with .30 cartriges that line up with the anecdotal reports.
Just FYI, casing ejection is handled here https://github.com/CleverRaven/Cataclysm-DDA/blob/master/src/ranged.cpp#L1413, and there is currently no damage done to the casing on ejection. I assume if you see damaged casings in-game it’s because they got stepped on, shot, or run over.
Knowing what I do now, I’d probably set degradation to destroy cartridges after about 30 reloads as a stopgap, then look into scaling the damage based on round power to accelerate rifle casing degradation and allow weak pistol ammo to last longer. This isn’t a high priority for me though.
Luckily, having casings for more powerful rounds have a shorter lifespan is exactly what we want for game balance.