Is this what you're trying to do? I'm assuming that $SubsTags.Tags holds a hash table. I'm not sure if "true" (in your situation) is a string value or a boolean value (i.e. $true or $false).
$hash = @{
"ghs-compliance"="soc2"
"ghs-managedossupport"="true"
"ghs-managedrbac"=" true"
"ghs-managedmonitoring"="true"
"ghs-managednetworking"="true"
"ghs-subscriptiontype"="fully managed"
"ghs-managedbackup"="true"
"ghs-managedconfigmanagement"="true"
"ghs-managedpatching"=" true"
}
if ($hash["ghs-managedbackup"] -eq "true"){
Write-Host "It's true!"
}
else{
Write-Host "It's NOT true."
}