Well there's definitely no know total set. There is an action statement named Clear User Defined Items by Source. If you select iTunes it will remove all User Defined Text iTems and User Defined URLs which begin with iTunes.

ITUNESADVISORY was not written but if it's stored as a UDTI the above statement will get rid of it.

That doesn't matter. Yate sees all tags. Where are you seeing the tag in Yate? On the UD Text editing panel or elsewhere? Is it possible that you're only seeing the tag in another tag editor? Perhaps that's how they display the Content Advisory field. Make sure that Preferences - Apple Apps - General does not have Write empty content advisory checked. Then clear the Content Advisory field on the More Info editing panel. If that does it the mystery is solved. However, there are some other players which supposedly support the field for Clean and Explicit settings.

If it doesn't work ... select a file which has the tag and do a View>Raw Data. Save and zip up the file and email it to support@2manyrobots.com

Content advisory is stored in a low level atom named 'rtng'. Your file has a 'rtng' atom with a value of 0. When you open the file in Yate it will not display a Content Advisory as the value is 0. However, it will not remove the current empty value unless the file is saved. Saving the file will remove the empty 'rtng' atom unless you have Write empty content advisory checked in Preferences-Apple Apps. The reason that some people want the option is to force iTunes/Music to update the advisory. If it's not in the file, it's not updated.

You can always do a File>Save Selected Files>Save Always or use a Save action statement with Save Always set. However, there supposedly are other players which use the setting and it only occupies 25 bytes in the file. It's up to you.

Well I can make it a little better. I can get it down to one write/read per folder to determine which files have to be processed. The file can be written to the same location for all folders and can be on an internal drive.

Basically the Log statement can save the raw data to a file. When running through the Batch Processor, Yate could write the file once per folder. It could then read and parse the file to firstly locate MPEG-4 files and then then to check if the 'rtng' atom exists. Then it would only do a force save on those file.

If you want to do this let me know and I'll knock off the action. It should be relatively easy.c

Download and import the file at:

https://2manyrobots.com/actions/SaveOnlyMPEG4WithRtng.zip

The action's name is: Save only MPEG-4 Files with rtng Atoms and is documented.

You can try it by loading some tracks into the main window and selecting them then running the action. The action also works in the Batch Processor. I'd try it on a subset of your collection before running your entire collection through the Batch Processor. I've done a little testing and it seems as if the mod dates are not changing where they shouldn't be.