Can't turn off "Visible on New Button" for Content Type using PnP PowerShell

Darren Parkinson Try Invoke-PnPQuery, you dont need to use ctx at all.

Working Code for me (I'm using the opposite approach, telling the function what content type I want to have in the "New" menu):

function SetVisibleContentType($listName, $ctName) {
    $ct = Get-PnPContentType -List $listName -Identity $ctName

    $list = Get-PnPList -Identity $listName -Includes RootFolder.UniqueContentTypeOrder

    $idList = New-Object "System.Collections.Generic.List[Microsoft.SharePoint.Client.ContentTypeId]"
    $idList.Add($ct.Id)

    $list.RootFolder.UniqueContentTypeOrder = $idList
    $list.RootFolder.Update()

    Invoke-PnPQuery
}

If you want to show more than one content type, just pass a string-array of content type names, or change the logic to show all but the provided conten type name.