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.
Floorp is an excellent browser. I mainly use it for the first class vertical tab support, but it’s ability to pin websites to open in a sidebar is great. I use it for stuff like Proton mail and drive.
I will say in case anyone here saw it in the past, it had a bit of drama around the author making part of it closed source, but that was fortunately resolved before it was ever released with any proprietary code.