Vivaldi 8.1 still supports it (Chromium 150) as loaded unpacked extension.
Edit: and latest August 14, 2026 Chromium 152.0.7977.38 based Snapshot https://vivaldi.com/blog/desktop/address-field-calculator-an... has this in patch notes:
"[Extensions] Re-enable and extend Manifest V2 extension support for the time being (VB-130324)"
I wouldn't be sure this is a long term solution. Maintaining a this into perpetuity would probably take a lot of resources that Vivaldi team might not have.
Unless upstream introduces breaking changes I don't see why not. Google was killing this functionality for policy reasons, not because changes to the renderer required it or something. And LLM analysis is going to make maintaining forks easier going forward.
For all possible MV2 extensions this is unsustainable for a small company. Supporting just uBO used by tens of millions of people? pretty doable as it only relies on few hooks into C functions that arent going anywhere.