This is a new feature from blogdown 0.18: https://github.com/rstudio/blogdown/blob/master/NEWS.md#changes-in-blogdown-version-018
It will use by default now whoami::fullname() if the whoami package is install.
You can set another default value using blogdown.author R option. (set it to "" if you want no value.
This is also a new behavior closer to Hugo best practice: blogdown will use Hugo's Page Bundle feature to organize the website now (It was opt-in before, but since blogdown 0.21, this is the default now). See about Page bundle:
We advice to use Page Bundle now as it has proven a better way to organize resource in a Hugo website, hence the default. If you really don't want Page bundle for your website, you can deactivate by setting option options(blogdown.new_bundle = FALSE). If you want to convert your old site not using page bundle yet to full page bundle use, there is a function for that to: blogdown::bundle_site()
I believe this is not related directly to R version. It is just that when you upgraded to R 4.0.3, you must have installed blogdown, and it installed the last version which lead to different version used for your website when you switch R version. Check the version in each R session using xfun::session_info("blogdown").
There are a lot of improvment in blogdown 0.21 and in the new version to come. You should read the NEWS file to get up to date with those.
We'll update the book and communicate on those changes in the RStudio blog very soon !
Thanks for your feedback by the way. That helps us see what is unexpected for users.