AFAIK they are using the built-in support which Google still maintains even if they don't expose it.

My point is that when MV3 was announced there was an assumption that maintaining a Chromium fork to continue supporting MV2 would be too much work and too expensive. But now with LLMs that's no longer a safe assumption. The non-Chrome chromium based browsers can fork chromium and add back MV2. No need for any expiration date.