How to get the index of $Matches index that satisfy certain criteria?

I think you want to output the filenames of all jobs where less then 30 frames where encoded? Is this what you're trying to do?

$content = Get-Content -Path "C:\log.txt"
$index = 0
$indexes = ($content | Select-String -Pattern 'encoded' | foreach {
    if($_ -match "(.*) ([0-9]*) (.*),(.*)") {
        # "remember" the indexes of the matches
        if ([int]$Matches[2] -lt 30) {
            $index
        }
        $index++
    }
})
$index = 0
$content | Select-String -Pattern 'Input' | foreach {
    if($_ -match "(.*) '(.*)':") {
        # output if index matches
        if ($indexes -contains $index) {
            $Matches[2]
        }
        $index++
    }
}

Alternative (shorter) solution:

get-content "C:\log.txt" | foreach {
    if ($_ -like "Input*" -and $_ -match "(.*) '(.*)':") {
        $file = $Matches[2]
    }
    if ($_ -like "encoded*" -and $_ -match "(.*) ([0-9]*) (.*),(.*)" -and [int]$Matches[2] -lt 30) {
        $file
    }
}

Please try if this works with your actual input files.

Also, I think your regex patterns could be improved, but for now it seems they do the job.

There is a misconception in the question. $matches[2] would not be an array of values you need to index. It would represent a scalar value for each interation of the foreach-object block. Those results are all sent down the pipeline individually.

I would still like to answer the question as stated though. Given an array of values, determine which of those is below a threshold and return the index of its position in the array.

$results = $content | Select-String -Pattern 'encoded' | 
    Where-object{$_ -match "(.*) ([0-9]*) (.*),(.*)"} | 
    ForEach-Object{$Matches[2]}

This is similar to the code you show above. It saves the values into an array called $results.

$threshold = 30
for($index=0; $index -lt $results.count; $index++){
    if([int]$results[$index] -lt $threshold){
        Write-Host "The value at index $index is $($results[$index]) which is below $threshold"
    }
}

Then we cycle the array elements, one by one, using a counter. Check every value and report the index we were currently at.

Given the scenario, I would go a different route altogether

I would like to take this in a slightly different direction. Understanding that each file with have multiple blocks like you show in the question that will correspond to multiple files you are processing and you are only interested in files where there are less than 30 encoded frames.

Note: This solution is heavily dependent on how the real data looks. I can only go on based what is in the question. If the real data deviates too much from that the solution is not guaranteed to work or give expected results.

Using [regex] lets split the file into its chunks. Using names capture groups we can pull just the values from each "block" that you want. Non-greedy captures will ensure we don't match outside of any given block. Then we make custom objects that we can filter just like you would normal PowerShell objects.

If the file called C:\log.txt looked like this:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'C:\myvideo.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf54.63.104
Output #0, mp4, to 'C:\output\myvideo.mp4':
  Metadata:
    encoder         : Lavf58.15.100
    Stream #0:0(und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(progressive), 864x480, q=2-31, 12800 tbn (default)
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp (default)
    Metadata:

encoded 2058 frames, 1376.59 fps, 373.36 kbps, 3.66 MB

Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'C:\myvideo2.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf54.63.104
Output #0, mp4, to 'C:\output\myvideo2.mp4':
  Metadata:
    encoder         : Lavf58.15.100
    Stream #0:0(und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(progressive), 864x480, q=2-31, 12800 tbn (default)
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp (default)
    Metadata:

encoded 0 frames, 1376.59 fps, 373.36 kbps, 3.66 MB

We could run this:

$content = Get-Content -Path "C:\log.txt" -Raw

[regex]::Matches($content,"(?sm)Input #(?<number>\d+).*?from '(?<filename>.*?)'.*?encoded (?<frames>\d+)") | ForEach-Object{
    [pscustomobject]@{
        Index = $_.Groups["number"].Value 
        Filename = $_.Groups["filename"].Value 
        EncodedFrames = [int]$_.Groups["frames"].Value 
    }

}

On its own would return

Index Filename        EncodedFrames
----- --------        -------------
0     C:\myvideo.mp4  2058         
1     C:\myvideo2.mp4 0          

So lets filter that output. Add onto the last line the following i.e. after the Foreach block ending parentheses: | Where-Object{$_.EncodedFrames -lt 30} and you would get just the ones you want. Then you could add | Select-Object -expand Filename to just get those file names.

All together now

$content = Get-Content -Path "C:\log.txt" -Raw

[regex]::Matches($content,"(?sm)Input #(?<number>\d+).*?from '(?<filename>.*?)'.*?encoded (?<frames>\d+)") | ForEach-Object{
    [pscustomobject]@{
        Index = $_.Groups["number"].Value 
        Filename = $_.Groups["filename"].Value 
        EncodedFrames = [int]$_.Groups["frames"].Value 
    }

} | Where-Object{$_.EncodedFrames -lt 30} | Select-Object -expand Filename