The current model for outdated raid content transitioning to Legacy loot rules versus personal loot is roughly 2 years.
The difference between the two are as follows.
Legacy loot allows a single player that has killed any boss of a tier to award 5 pieces of loot. This items will be of any item listed on that bosses loot table.
Mail, plate, cloth, leather, weapons, trinkets, rings.
Personal loot will drop loot based off a system introduced a few years ago where 1-2 pieces of loot per 5 people in the raid group at the time of the death of the boss.
This scales up to 20 players = 5-6 pieces of loot depending on the instance/ boss.
(Castle Nathria in mind where an additional loot may be awarded based on criteria I cant remember or currently know).
There is an ever growing amount of players that have aspirations of collecting tier sets, weapons and offset pieces for Shadowlands raids. The only method to farm them currently is to join a full raid group and roll on tmog. Which can take months for even one class set.
Many players are able to solo most, if not all LFR, normal and heroic raids solo. Yet this can lead to zero items from every boss. Since there are not at least 5 members in group to provide at least one piece of guaranteed loot for personal loot rules.
The rate at which these legacy changes are made are desired by many to be shifted from long term.