How to Disable Modern Experience in SharePoint 2019?
By running the script below, you can disable Modern UI on the SharePoint server at the Site Collection Level.
add-pssnapin Microsoft.SharePoint.PowerShell
$site = Get-SPSite {siteURL}
$featureGUID = New-Object System.Guid "E3540C7D-6BEA-403C-A224-1A12EAFEE4C4"
// Disable Modern Experience
$site.Features.Add($featureGUID, $true)
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How to Enable Modern Experience in SharePoint 2019?
Run the same script for Enable Modern Experience, but remove the GUID from Features.
add-pssnapin Microsoft.SharePoint.PowerShell
$site = Get-SPSite {siteURL}
$featureGUID = New-Object System.Guid "E3540C7D-6BEA-403C-A224-1A12EAFEE4C4"
// Enable Modern Experience
$site.Features.Remove($featureGUID, $true)