See https://jekyllrb.com/docs/upgrading/3-to-4/

The post_url tag now incorporates the relative_url filter within itself and therefore automatically prepends your site's baseurl to the post's url value.

This is done with

sed -i -z -E -e 's|\{\{ site.baseurl \}\}/?([{%\\n[:space:]"]+post_url)|\1|g' _docs/*.md _posts/*.md po/*.po po/*.pot