The underlying requirement (except summons) for publication on the website of agenda and minutes is Section 2, subsection 1A of the Local Government, Planning and Land Act 1980.
It was introduced in 2014 by the Local Audit and Accountability Act 2014.
In the case of Parishes, it is the Local Transparency Code.
Section 3 of the LGPL1980 is the element that makes anything in the code that says "must" become statutory.