All promises regarding updates are valid from the date the device was introduced to the market. The date of purchase has no influence at all. The OnePlus 8 Pro was launched in April 2020 and has an ancient SnapDragon 865 processor compared to new devices.
But why spend so much money when you can get almost the same thing for less? The Xiaomi 11T is guaranteed 4 years of updates. That's better than 3 and it costs less than 600 francs with better technology.
Xiaomi 11T (256 GB, Meteorite Gray, 6.67 ", Dual SIM, 108 Mpx, 5G)And even the best smartphone from Xiaomi, the 11T Pro, costs just 700 francs and that also has 4 years of guaranteed updates and the fastest SnapDragon 888 processor.
Xiaomi 11T Pro (256 GB, Meteorite Gray, 6.67 ", Dual SIM, 108 Mpx, 5G)