@ipstenu – Thanks for correcting me.
Now, for Multisite I guess i installed Sitewide plugin (but don’t remember now, as I have installed it 8-9 months ago). I did search (before starting this thread) for installed plugins to check if sitewide is being used in my Multisite installation, but not able to find it.
After this I was looking for plugin files in installation. And found sitewide-tags.php in mu-plugins folder.
Now, how should I proceed further.