UTF-8 encoding problem when merging txt files in Powershell

In PowerShell < 6.0, the Out-File cmdlet does not have a Utf8NoBOM encoding.
You can however write Utf8 text files without BOM using .NET:

Common for all methods below

$rootFolder = 'D:\test'  # the path where the textfiles to merge can be found
$outFile    = Join-Path -Path $rootFolder -ChildPath 'newfile.txt'

Method 1

# create a Utf8NoBOM encoding object
$utf8NoBom = New-Object System.Text.UTF8Encoding $false  # $false means NoBOM
Get-Content -Path "$rootFolder\*.txt" -Encoding UTF8 -Raw | ForEach-Object {
    [System.IO.File]::AppendAllText($outFile, $_, $utf8NoBom)
}

Method 2

# create a Utf8NoBOM encoding object
$utf8NoBom = New-Object System.Text.UTF8Encoding $false  # $false means NoBOM
Get-ChildItem -Path $rootFolder -Filter '*.txt' -File | ForEach-Object {
    [System.IO.File]::AppendAllLines($outFile, [string[]]($_ | Get-Content -Encoding UTF8), $utf8NoBom)
}

Method 3

# Create a StreamWriter object which by default writes Utf8 without a BOM.
$sw = New-Object System.IO.StreamWriter $outFile, $true  # $true is for Append
Get-ChildItem -Path $rootFolder -Filter '*.txt' -File | ForEach-Object {
    Get-Content -Path $_.FullName -Encoding UTF8 | ForEach-Object {
        $sw.WriteLine($_)
    }
}
$sw.Dispose()