I'm not sure you are right about that, but it was uor that made GC quit, there was some changes that forced mages to changes their template, can't remember all of it.
I still remember uor as the public that killed tankmages.
I can't find other than this changes:
Changes UOR (what brought Trammel and Siege) to Ultima Online were as follows:
UOR
No more precasting.
When you casted a spell your spellbook would automatically be equipped.
Mindblast now did damage based on the spread of your stats. So if you were 100str 100int/dex and 25in whatevers left... you get hit for crazy damage... 40+ which back then was crazy damage for a quick 5th circle spell. (This was an era where I would carry a wizards hat which altered stats with me.)
Special moves became a part of the fighting skills: fencing (paralyze,) macing (crushing,) and swords (concussion.) And it was for only certain weapons... for example: a spear would do the special move, but a kryss wouldnt.
The chance to do a special move depended on your intelligence. (Higher int = higher chance.) They went off randomly (RNG) and not by the push of a button.
AR (Armor's) range was from 0 to 70 just like it is today... the only problem was-was that damn near no one knew how to get 70AR (cept a few) and it was by wearing indestructable/invulnerable magic plate armor w/ indest/invulnt shield.
Think of it like this... AR = Physical Resist and having GM Magic Resist automatically gave you in todays terms 60 in all elemental resists. (The big reason I've always lobbied for going back to a similar system. Ironic how I've gotten the strongest kickback from those who claim they loved PreAoS the most!!!)
Regular GM plate suit gave you 40? or was it 44? - AoS is what changed our armor formulas. The only bug that was fixed, was with UOLBR where you could wear chain mail legs under your plate suit. (Something I abused pre-UOR to run a suit of 70AR with a DP spear and over 500 charges of magic reflect... ie, it made me damn near invincible.)
UO LBR (Lord Blackthornes Revenge) brought a few more changes.
Casting spell no longer auto-equipped spellbook.
Harm, Fireball, Lightning, and Magic Arrow now did damage based on distance (not the circle of spell they were.) Meaning the closer to your opponent you were, the more damage they did to them.
Mindblast was changed to be capped. Still worked the same way, but wasn't as bad as UOR.
Somewhere in between UOR and LBR this was changed: Anat + Wrestling = stun punch special move. (Heal mages became quite popular.) and Arms Lore + Wrestling = disarm (disarm thiefs come to mind.)
The events that brought us Factions, savage paint, bolas, orcish bows and tribal spears came about.
Which sums up that era of UO.
The funny thing is that Siege... is a UOR shard. Not a pre-UOR shard (with precast and halberd mages.)