Lightroom Classic: Edit date

Thanks Todd, I didn't know about this one.
Are you using it ?
Do you get the results int the LR filter window or a seperate window ?

John, thank you for the clarification!

I'm in a similar process. While I reprocess many old shots based on LR and skill improvements, I have also encountered a number of images that had their original adjustments trashed - it looks like at the time of metadata updates in the history panel. Lens profiles, all detail settings, all basic settings were zeroed. Not sure how it happened but could be related to my trying Capture One and they dorked my catalog, which definitely royally screwed up things. I was able to restore my LR catalog to the one before their process trashed it since I did a backup, but there were a number of XMP files that were really hosed and their changes overwrote my LR edits. (Just a warning for others tempted by promises of greener pastures.)

I noticed something similar many years ago with a lot of older image files having no adjustments. I'm fairly certain I had adjusted many of them so I'm guessing one of the LR upgrades (1>2>3>4>5) wiped out the settings. I've been using LR since 1.0 so it's not surprising something like this happened. BTW- I've never imported my LR catalog into another raw editor like Capture One. It looks like the LR 3.0 upgrade (June 2010) may be the culprit when PV 2010 introduced, which is now called Process Version 3. Files imported after that date don't appear to be missing edits.
To find the files with missing adjustments and those shot with the earlier Process Version 1 and 2 I used the below Any Filter settings. Notice I have 'Save results as a collection' checked, which will allow you to do additional Library Filter searches on the collection such as by Date or Camera. PV 3 was the first process version to introduce the image adaptive Tone controls so any images that have been edited with PV 3 or 4 will see the least benefit from updating to PV 5. You probably want to do that on an "as needed" basis when you revisit those images for output.

For me, this would have been LR 8.x. I estimate about 1/3 of my files might be affected and there are some common dates - 10/18/2018 and 6/1/2019 are ones I've looked at which impact multiple files. Your theory about an LR update is still a good one, I update often (and I'm on the beta program so more often than most). Another possible source is syncing, still looking into that one. I stopped using LR Desktop and rarely use LR on my iPad. And turned off sync but it seems to turn itself on again.

PV doesn't matter so much as nearly all adjustments are gone anyway.

With lots of time (I'm retired anyway), I have been doing a lot of reprocessing because I'm smarter now :) I did notice this months ago and was only tackling images as I worked on my flickr collections. 

Make that 11-30-2018, not 10-18. Which corresponds with L 8.1 released the day before.

LR 8.0 (10/18/2018) introduced Process Version 5, and it automatically updated photos to Process Version 5 if you took them into Develop (even if you didn't make any changes). Over the years, there have been a number of problems with such auto-updating of various fields, so perhaps your 11/30/2018 dates were from another instance.

One other suggestion I forgot to add is that you can batch apply the new PV 5 Adobe Sensei Auto Tone function to speed the editing process. Don't forget to update the image to PV 5 as the first step. More here:

https://community.adobe.com/t5/lightroom-classic/automatically-adjust-hundreds-of-photos/m-p/1102375...

https://community.adobe.com/t5/lightroom-classic/automatically-adjust-hundreds-of-photos/m-p/1102599...

Agreed.