When there has been clear rules regarding warfare the ooc problems were far smaller. I'm fact, I'd go so far as to say the biggest reason IC conflicts spill over to ooc to such extent that people leave is because of the lack of rules allowing for war. Again I point to the Game of Crowns. We had rules for that. Arcturus and Arduin lost, but we only had a few people leave during that event, and those were more from frustrations between those players and staff rather than because they lost.
Getting players invested in something bigger than themselves is a sure fire way to keep our population strong, and I've see no better than organizations like houses, and houses need a reason for being. Game of Crowns does that.
Having been around for both GoCs, no, not really. We lost the entirety of the Arduin playerbase due to the rules being written
by and in favour of the folks that won the first round, and those war rules were straight MC PVP.
Now in the days of enchantments and potions and whathaveyou, MC PVP is even less workable.
DMed combats don't work the instant you have one DM and two sets of people flanking, and a lot of actions go missed because one has difficulties auditing the whole battlefield.
The only way to have workable mass war at this point is to have every single combatant obey a single turn order of emotes and have turnbased rules for the whole thing, with the whole thing audited by the DM(S) (You need plural, trained people, for that), and have everyone prepare for the battle to take most of a day.