What happened to the Nikon Z6 Camera Matching Profiles?

Adobe has acknowledged the bug:

https://feedback.photoshop.com/photoshop_family/topics/camera-specific-profiles-missing-once-updating-from-lightroom-classic-9-1-to-9-2

Please add your constructive opinion to the bug report, and be sure to click Me Too and Follow in the upper-right corner. That will make it a little more likely that Adobe will prioritize a fix, and you'll be notified when the bug's status changes.  

Adobe rarely indicates ahead of time when a bug will be fixed, but new versions come out about once every two months.

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Just a note to express my gratitude for the researched responses I received for this post. Thank you.

I'll wait to see what Adobe do with the support request.

"I'm just curious as to why Adobe dropped them."

It appears to be an unintentional bug. Those "picture control" profiles are available for the Z 7 in LR 9.2, and the .xmp files defining them for the Z 6 are present in LR 9.2, e.g. on Mac:

/Library/Application Support/Adobe/CameraRaw/Settings/Adobe/Profiles/Camera/Nikon/Nikon Z 6/Nikon Z 6 Camera Melancholic.xmp

But LR 9.2 isn't reading them for some reason, though LR 9.1 did.

I recommend you post a bug report in the official Adobe feedback forum, where Adobe wants all bug reports and feature suggestions: 

https://feedback.photoshop.com/photoshop_family/categories/photoshop_family_photoshop_lightroom

Be sure to include the first ten lines of the menu command Help > System Info.  Provide all the details, rather than linking back to this thread -- experience shows that Adobe developers tend not follow the links.

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Not a holder of a Z6, so downloaded a sample. Brought it into Develop, and the following screenshopt shows 11 Camera Matching Profiles.

Now looking online,  (  source ) I see that the Nikon Z6 comes with Picture Controls, the normal ones being: Standard, Neutral, Vivid, Monochrome, Portrait, Landscape and Flat. And those are included (with 5 Monochromes)

Did the Camera Matching profiles in previous version of LRC include the " Gimmick Picture Controls" (Creative Picture Controls), those being:Dream, Morning, Pop, Sunday, Somber, Dramatic, Silence, Bleached, Melancholic, Pure, Denim, Toy, Sepia, Blue, Red, Pink, Charcoal, Graphite, Binary and Carbon????

" Gimmick Picture Controls", not my words, that is from the source I listed above

Follwup/ I see in the online manual, Nikon calls those Creative Picture Controls.

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I imported a sample Z 6 from dpreview.com into LR 8.2.1, and it shows the same 11 Camera Matching profiles as for my LR 9.2:

So I think your current LR installation is fine. Without seeing a screenshot of what you were seeing before, it's hard to know why you saw an abnormally large number of Camera Matching profiles.

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With the update to Lightroom Classic 9.2 Adobe has made introduced new procedures for creating Default Develop settings for individual camera models. You can see info about this in the Top post in this forum. The link below also provides additional details on how to proceed with the process.

https://blogs.adobe.com/jkost/2020/02/how-to-customize-lightroom-classics-default-develop-settings-v9.html