EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
Brave is forked from Chromium so hypothetically they could maintain V2 but they’d need their own store as they currently rely on Googles
Guess we will see if Brave is all talk or not in the coming years.