Only if msExchHideFromAddressLists is missing from: AD > User > Properties > Attribute Editor

Start > Synchronization Rules Editor
In from AD - User join > Edit > Yes
Precedence: 51 (or higher, up to 99)
Transformations > Add transformation
FlowType: Expression
Target Attribute: msExchHideFromAddressLists
Source:

IIF(IsPresent([msExchAssistantName]),IIF([msExchAssistantName]="HideFromGAL",True,False),NULL)

PowerShell:

Start-ADSyncSyncCycle -PolicyType Initial

HideFromGAL

Start-ADSyncSyncCycle -PolicyType Delta

$OU = "OU=Archive,OU=Customer,DC=DOMAIN,DC=local"
$AttributeValue = "HideFromGAL"
$users = Get-ADUser -SearchBase $OU -Filter * -Properties msExchAssistantName

$total = @($users).Count
$updated = 0
$skipped = 0

Write-Host "Found $total user(s) in $OU" -ForegroundColor Cyan

foreach ($user in $users) {
    if ($user.msExchAssistantName -eq $AttributeValue) {
        Write-Host "  SKIP: $($user.SamAccountName) - already set" -ForegroundColor DarkGray
        $skipped++
        continue
    }

    try {
        Set-ADUser -Identity $user -Replace @{ msExchAssistantName = $AttributeValue }
        Write-Host "  SET:  $($user.SamAccountName)" -ForegroundColor Green
        $updated++
    }
    catch {
        Write-Host "  FAIL: $($user.SamAccountName) - $_" -ForegroundColor Red
    }
}

Write-Host "`nDone. Updated: $updated, Skipped: $skipped, Total: $total" -ForegroundColor Cyan
Write-Host "Next step: Run 'Start-ADSyncSyncCycle -PolicyType Delta' on the Azure AD Connect server." -ForegroundColor Yellow