I would prefer this to be following so that the logs on failure have good error message.

throw "No Exception thrown when FileNotFound,Microsoft.PowerShell.Commands.UnblockFileCommand was expected"

I have two "cons" (for all comments).
I believe we should follow the documentation (throw "No Exception!" ) and preserve consistent in all the tests that currently use this docs template. If a template is bad, then we should change it and bring all the tests to the new template.

If Unblock-File test returns FileNotFound, then this will not help our understanding of the problem's root and why the exception throw. (We should mention a Zone.Identify stream and give a link to a MSDN article in the message.) I believe this is redundant, since in any case we will always start with the analysis of the test code and do more manual tests. So I prefer to see Expected : {True} But was : {False}. that simple means that a file is not unlocked.

I am fine with this being kept as "No Exception"

But as of the function returning a $true or $false, I would prefer it to be like below as the error will give us the line number where the should is located :

function Test-UnblockFile {
    try {
        Get-Content -Path $testfilepath -Stream Zone.Identifier -ErrorAction Stop | Out-Null
        throw "No Exception"
    }
    catch {
        $_.FullyQualifiedErrorId | Should Be "GetContentReaderFileNotFoundError,Microsoft.PowerShell.Commands.GetContentCommand"        
    }
}

It "With '-Path': file exist" {
        Unblock-File -Path $testfilepath
        Test-UnblockFile 
    }

    It "With '-LiteralPath': file exist" {
        Unblock-File -LiteralPath $testfilepath
        Test-UnblockFile 
    }

If we move Should to a function we get line number in the function, don't we? It seems this is not what we want.
And I am waiting #2973

@adityapatwardhan Issue #2973 was closed. Please continue the review.

Same as the other comment.

I am fine with this being kept as "No Exception"

If this test fails, the error message would be Expected : {True} But was : {False}. This does not help in debugging the failure. I would prefer this to be like:

Unblock-File -Path $testfilepath
try {
    Get-Content -Path $testfilepath -Stream Zone.Identifier -ErrorAction Stop | Out-Null
     throw "No exception thrown when GetContentReaderFileNotFoundError,Microsoft.PowerShell.Commands.GetContentCommand was expected"|
}
catch {
    $_.FullyQualifiedErrorId | Should Be "GetContentReaderFileNotFoundError,Microsoft.PowerShell.Commands.GetContentCommand"
}

Same as above.

I am fine with this being kept as "No Exception"

But as of the function returning a $true or $false, I would prefer it to be like below as the error will give us the line number where the should is located :

function Test-UnblockFile {
    try {
        Get-Content -Path $testfilepath -Stream Zone.Identifier -ErrorAction Stop | Out-Null
        throw "No Exception"
    }
    catch {
        $_.FullyQualifiedErrorId | Should Be "GetContentReaderFileNotFoundError,Microsoft.PowerShell.Commands.GetContentCommand"        
    }
}

It "With '-Path': file exist" {
        Unblock-File -Path $testfilepath
        Test-UnblockFile 
    }

    It "With '-LiteralPath': file exist" {
        Unblock-File -LiteralPath $testfilepath
        Test-UnblockFile 
    }