I think both was 5 years ago and was planned all along by Doom and Max.
Mercy was left in Havana to help the injured
Winston had to build a new chronal accelarator and find Tracer
Genji had to be repaired from the fight against Doom
Reinhardt left Overwatch
Ana was in presumed dead status
We dont know what was with Torb, Moira and McCree
So OW was an easy target with Jack and Reyes dividing the remaining forces.
As public distrust swelled, a massive explosion wiped out Overwatch’s headquarters in Switzerland. The United Nations called it an accident; today we know it was a battle, a dispute between Overwatch commander Jack Morrison and Blackwatch commander Gabriel Reyes. Overwatch’s death knell was that of two former comrades laying waste to all they had built.
In the wake of such an incident, little could remain hidden. The full transgressions of Overwatch’s shadow operations became known. Even the most ardent defenders inside the organization bowed to the truth and called for its dissolution.
This shows us that the Swiss HQ explosion wasn’t the end of Overwatch - it continued on after that until it was formally closed by the Petras Act.
When he died under mysterious circumstances six years ago, I felt as if a part of my childhood died with him.
As for the date of when Overwatch was disbanded, Michael Chu has answered that:
So, the rough timeline is as follows:
9 years ago: The Ecopoint: Antarctica team is hit by a massive ice storm and forced into cryosleep to wait it out. Reyes hires Moira.
8 years ago: The Venice Incident; Reyes kills Antonio and Blackwatch is exposed to the world and the inquiry into Overwatch is started. Tensions begin to grow between Reyes and Morrison.
7 years ago: The King’s Row Uprising; Overwatch steps in despite being told not to go. They rescue the hostages and seem to be regaining some credibility.
6 years ago: Morrison and Amari go on a mission in Poland to rescue some kidnapped scientists; Ana is engaged by Widowmaker and appears to lose the fight. Reyes sends McCree to look for her, but he fails. Also, the Havana Mission; Overwatch strike team apprehends Maximilien to find Doomfist’s location. 3 weeks later, they corner him in Singapore and Doomfist is arrested; Tracer and Genji are injured. Some time after this, Morrison and Reyes come to blows in a fight that results in the Swiss headquarters building exploding (the public believes they caused it; Morrison believes it was sabotage from someone on the inside).
5 years ago: Overwatch is disbanded by the UN. Morrison, Reyes and Amari are believed to be dead, McCree and Genji have already left, and Moira is a public disgrace.
Oasis hired her despite her reputation - they didn’t care what she had done.
Angela did testify against OW - she thought Reyes and Morrison had fallen out over Morrison being promoted over Reyes, which we know isn’t true (she wasn’t a member of their inner circle and just assumed).