It is called "Side Initiative". It is a variant of the initiative on page 270 of the DMG.
It states
"If you want quicker combats, at the risk of those combats becoming unbalanced, try using the side initiative rule. "
Side initiative is roll Initiative without using modifiers. One roll, for an entire party. NO MODIFIERS. Meaning aside from balance, it also breaks quite a few items and skills and even a few spells and feats and the dex ability bonus to initiative too.
So to me that was always a red-flag to never use it... I am really curious how Larian is gonna explain that worry away in the AMA, because that it breaks balance is what it says on the DMG page 270
Funnily enough it does not say why.
