System.IO.Compression.DeflateStreamで圧縮した.deflate拡張子のファイルを放り込むだけでとりあえず展開できるやつ。Windows10上のPowerShell 7.1.2で動作確認した。

Decompress-Deflate.ps1

param(
    [string]$file
)
if ([System.IO.Path]::GetExtension($file) -ne ".deflate") { return; }
$outfile = [System.IO.Path]::Combine([System.IO.Path]::GetDirectoryName($file), [System.IO.Path]::GetFileNameWithoutExtension($file));
try {
    $infs = New-Object System.IO.FileStream($file, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read);
    $ds = New-Object System.IO.Compression.DeflateStream($infs, [System.IO.Compression.CompressionMode]::Decompress);
    $outfs = New-Object System.IO.FileStream($outfile, [System.IO.FileMode]::CreateNew, [System.IO.FileAccess]::ReadWrite);
    $ds.CopyTo($outfs);
}
finally {
    $infs.Dispose();
    $ds.Dispose();
    $outfs.Dispose();
}
Write-Output $outfile;