It’s just unscientific thinking. People think virus and bacteria are the only thing you have to worry about, but lots of the time it’s the bacteria producing toxins as part of their metabolism that’s dangerous to us. In other words, their shit is poison.
One of the reasons we don’t want some groups of bacteria growing on our foodstuff is because they turn stuff literally toxic to us, completely unrelated to immune responses. Same way some molds can be toxic while others are not. It’s not because the fungus starts growing inside your body and has an epic free for all with your immune system. Its byproducts are just toxic. Like some berries or some plants are toxic.
They are deprecating the underlying technology(called manifest V2 or MV2 for short) and replacing it with a different one(MV3) that lacks some of the capabilities for some kind of adblocking.
So yeah, it’s pretty much dead on chromium. The developers of brave have commited to provide a best effort support for their browser though: https://brave.com/blog/brave-shields-manifest-v3/
Firefox on the other end has no intention of deprecating support for MV2 so any browsers based on that are fine. Keep in mind MV3 supports some adblocking and some Adblockers have already moved to it, it’s just a lesser extent.