What is paid SEO?
As usual in the SEO sector, there are dissenting opinions.
According to some, paid indexing corresponds to sponsored links sold by the search engines.
In my opinion, paid indexing corresponds to offers that don't seem actively proposed by the major search engines under the name "trusted feed" or "paid inclusion".
Yahoo had an offer called "Search Submit Pro"on which they wanted to communicate little.
As far as I know, Google never proposed such an offer.
It is way more common to see it on directories and professional resources where being listed may require an annual subscription.