As posted a few weeks ago, Microsoft completed the Private Channel migration at the end of April.
Microsoft has now updated the migration state. Compared to April, a new state may appear if the migration had issues: RequiresAdminAttention. You need the Teams Administrator role to run the cmdlet.

If such a state appears, Microsoft has added new details about the failed migration. Currently, a list of ownerless channels, which cannot be migrated in this state. The information includes the team/group ID and channel ID.

I have prepared a PowerShell sample to enrich information for ownerless channels, including team names, team owners, and channel names, making it easier to handle them internally.
# This PowerShell sample retrieves the list of ownerless private channels in a tenant, and prepares a report with the team and channel details to support follow-up actions.
Import-Module MicrosoftTeams
Connect-MicrosoftTeams
$TeamStatus = Get-TenantPrivateChannelMigrationStatus
$TeamStatusJSON = $TeamStatus.Details | ConvertFrom-Json
$TeamStatusJSON.ownerlessChannelsDetails
$TeamDetails = @()
$Count = 1
foreach ($channel in $TeamStatusJSON.ownerlessChannelsDetails) {
# Prepare variables for team and channel names, in cases where the teams or channels have been deleted already
$Teamname = "Team unavailable"
$Channelname = "Channel unavailable"
try {
# Get the team details. If the team has been deleted, this will throw an error which we catch to avoid breaking the script, and to log for follow-up
$team = Get-Team -GroupId $channel.teamId
$Teamname = $team.DisplayName
# Include team owners in the report, to make it easier to identify the right internal people to contact
$TeamOwners = (Get-TeamUser -GroupId $channel.teamId -Role Owner).User -join ", "
Write-Host "$Count of $($TeamStatusJSON.ownerlessChannelsDetails.count) - Team: $($team.DisplayName)"
try {
# Get the channel details. If the channel has been deleted, this will throw an error which we catch to avoid breaking the script, and to log for follow-up
$channelDetails = Get-TeamChannel -GroupId $channel.teamId -MembershipType Private | Where-Object { $_.ID -eq $channel.channelThreadId }
$Channelname = $channelDetails.DisplayName
}
catch {
Write-Host "Private channel unavailable for Team '$($team.DisplayName)': $($channel.channelThreadId)" -ForegroundColor Yellow
}
}
catch {
Write-Host "Team / M365 Group is unavailable: $($channel.teamId)" -ForegroundColor Red
$TeamOwners = $null
}
# Add the details to the report object
$TeamDetails += [PSCustomObject]@{
TeamName = $Teamname
TeamId = $channel.teamId
TeamOwners = $TeamOwners
ChannelName = $Channelname
ChannelId = $channel.channelThreadId
}
$Count++
}
$TeamDetails | Sort-Object TeamName 