In case anyone comes to this down the line...

Get the count and amount of all attributes from a user in AD - use the UI for this.

Once you have the details, then update the script as needed:

$ADusers = Get-ADUser -Filter "objectclass -like 'user'" -Properties *

$output = @()
$userCheckedCount = 0
foreach ($user in $ADusers) {
    $userCheckedCount ++
    Write-Progress -Activity "Checking user: $($user.name)" -Status "$($userCheckedCount) of $($ADusers.count)" -PercentComplete $($userCheckedCount / $ADusers.count * 100)
    $output += [pscustomobject]@{
        User                 = $user.name;
        OU                   = $user.DistinguishedName.Replace(',','.');
        Enabled              = $user.enabled;
        extensionAttribute1  = $user.extensionAttribute1;
        extensionAttribute2  = $user.extensionAttribute2;
        extensionAttribute3  = $user.extensionAttribute3;
        extensionAttribute4  = $user.extensionAttribute4;
        extensionAttribute5  = $user.extensionAttribute5;
        extensionAttribute6  = $user.extensionAttribute6;
        extensionAttribute7  = $user.extensionAttribute7;
        extensionAttribute8  = $user.extensionAttribute8;
        extensionAttribute9  = $user.extensionAttribute9;
        extensionAttribute10 = $user.extensionAttribute10;
        extensionAttribute11 = $user.extensionAttribute11;
        extensionAttribute12 = $user.extensionAttribute12;
        extensionAttribute13 = $user.extensionAttribute13;
        extensionAttribute14 = $user.extensionAttribute14;
        extensionAttribute15 = $user.extensionAttribute15
    }
}

$output | Export-Csv -Path C:\temp\adAttributes.csv -Delimiter ';' -NoTypeInformation