Add a field to the (sealed) content type "Folder" in a library.

AddFieldToFolderCT.ps1

# 1) unseal the content type for folders in the library

$ct = Get-PnPContentType -Identity "Folder" -List "Documents"
$ct.Sealed = $false
$ct.Update($false)  # using $true in this case will throw an execption
Invoke-PnPQuery


# 2) add the description field (must already exist) to the Folder content type

$ct = Get-PnPContentType -Identity "Folder" -List "Documents"

$field = Get-PnPField -List "Documents" -Identity "Description"

$flci = New-Object Microsoft.SharePoint.Client.FieldLinkCreationInformation
$flci.Field =$field
$ct.FieldLinks.Add($flci)

$ct.Update($false)
Invoke-PnPQuery