Mackal/Demonstar awhile back had decompiled older era clients and summarized the results on some github pages and is a must-read for any old EQ recreation devs. Can be found here:
https://github.com/mackal/EQMechanics/wiki
Recently the Quarm server has motivated us to look at pre-PoP mechanics, and Kicnlag from Project Lantern has decompiled several different older clients and discovered a few new things to add to Mackal's work. (notably the 405 warrior cap)
This is the AC cap timeline that we're aware of as of this post:
* Classic (pre-Kunark) had a hard mitigation AC cap of 350 for all classes.
* At Kunark launch, level 51+ warriors had their hard cap increased to 405. All other classes (and sub 51 wars) remained at 350. (verified from the April 28 2000 client)
* AC caps changed again some time after (or at) Velious' launch. The earliest known client having the higher AC caps is April 4 2001. The new caps are: 430, 403, 375 for level 51+ melee classes only.
* The hard caps turn into soft caps for melee classes only in the late Velious time frame. (or maybe Luclin launch, not sure; would be nice to narrow this down) The overcap returns at this time are divided by 12 for all melee classes.
* PoP launch made shield AC ignore the cap. Also class and level based overcap returns. Casters get softcaps at this time, so no more hardcaps for any class.