AFAICT in practice it's a mandatory requirement in I²C subsys (in the
past you hadn't accepted a patch from us without a tag from the
maintainer) which makes it equal to sending PR by a maintainer. PR

Right. I require a tag from the driver maintainer.

makes less burden since subsys maintainer don't need to run many tools
or a tool many times to get the pile of patches.

I had driver maintainers who found it difficult to have a public tree to
pull from or forgot how to send properly prepared pull requests. They
were happy to send Rev-by, though. And I am happy with that, too. At
least in I2C, picking up patches is small work compared to the actual
review.