As summary. Looking at qtdeclarative/src/quickcontrols/fusion/MenuItem.qml, the menu text colour uses "control.palette.text". On 6.4 this is "#bebebe" in the disabled state, but in 6.5 it's "#000000", i.e. the same as the active state. This is presumably something to do with the colours being derived from the GTK theme in 6.5+? I haven't checked styles other than Fusion but I would imagine they use the same palette colour.

6.4.3:

6.5.3:

import QtQuick
import QtQuick.Controls

ApplicationWindow
{
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    menuBar: MenuBar
    {
        Menu
        {
            title: "Menu"
            MenuItem { text: "Item" }
            MenuItem { text: "Disabled Item"; enabled: false }
            MenuItem { text: "Item 2" }
        }
    }
}