[…] why the <ref> tag parameter follow= is not turned off in wiki sites that are not wikisource?
As the Technical Wishes team at Wikimedia Germany is currently working on the Cite extension, I had a look to find an answer to your question.
The follow="…" feature was introduced in 2010[1] as part of the Cite extension, the extension responsible for the <ref> and <references> tags. The feature was never designed to be turned on or off per wiki, and can't be turned off.
In case a wiki does not want to use it, and make sure it isn't used, I suggest to either use an "insource" search[2] on a regular basis, or possibly talk to the users running the Check Wikipedia project[3] to add a scan for the follow="…" parameter to their list.
Best Thiemo
[1] https://phabricator.wikimedia.org/rECITb5883ddc36f8ee270f555ad73ac5fa00cf1bc... [2] https://www.mediawiki.org/wiki/Help:CirrusSearch#Insource [3] https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Check_Wikipedia