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.
I’ve been using LibreWolf on Desktop and Mull on Android. Basically more securely configured versions of Firefox with the proprietary telemetry and some other stuff removed. Takes some tweaking to get certain websites to work that need more access than they should or use Certificate Authorities that don’t have working OSCP servers.