How can I delete browser history even when parental controls are off, the button is greyed out

That indicates it is already deleted.

Baseball_2023 Said:

"How can I delete browser history even when parental controls are off, the button is greyed out"

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For reference...

Clear Temporary Internet Files:

Try clearing your Temporary Internet Files (Cookies & Cache) of Safari. Cookies perform remembrance of certain site-entered data. Whereas Cache keeps a snapshot of the last time you visited a site.  Use this Link: Clearing History and Cache - Apple Communities