P: Ability to sync Lightroom Classic keywords with the Lightroom Ecosystem

Keywords are plain text. They are simply parsed differently in different apps.

keyword a|keyword b|keyword c

keyword a
          keyword b
                    keyword c

I don't see why the keywords cannot be made the same between LR Classic and LR Mobile.  It's too difficult to do any more than moderate edits on the phone, but tagging is a great use of down time, if only they would sync back to Classic.  Heck, even if it doesn't have the same hierarchy, at least a tag can be set and easily re-tagged in classic.

I'd like to extend this "FEATURE REQUEST"  a bit. 

We need a way to designate options on Keywords in Classic that pertain to syncing.  Right now, when the Smart Collection used for syncing is first created (either by the user or automatically) all the keywords get included in the smart preview including all parent keywords and excluding all synonyms - regardless of which options are selected in the add/edit keyword screen.  What we need is the addition of a few more options on keywords such as "Sync this Keyword",  "Sync Synonyms",  "Sync Containing Keyword".  

One of the excuses Adobe has expressed in implementing keyword syncing is that keywords in LR Cloud are linear whereas in LR Classic they are hierarchical.  One solution would be to make them hierarchical in the LR Cloud eco-system as well but that may be too much to ask. 

Assuming they stay linear in one and hierarchical in the other, presents a problem as the same Keyword can appear in several hierarchies in Classic.  For example "Red" could be under "Birds" and under "Cars" in LR Classic but in LR Cloud there could only be one which would make syncing problematic.  Without going into all the logic permutations, one solution  is that in LR Classic, whenever you mark a Keyword to Sync, it checks to assure that another keyword with the same name but under a different parent is not also marked to Sync.  If so, it would deny the request to mark it for sync.  In this way, there would no longer be a case where a single keyword in LR Cloud could map to more than one keyword in LR Classic.  In our example, if one wanted both occurrences of "red" to sync, the user would need to rename one or both of them (e.g.  Red-Bird and Red-Car).

Jileen, there are two ways to do what you want:

1. First and fastest even though it is more steps is to import through Classic setting the new imports to sync. This will upload smart previews of the images to the cloud. If this is done, then open up Lightroom CC on your desktop and import the same images. Lightroom CC will sync the full raw images to the cloud and link them to the smart previews already there so you won't get duplication doing this.
2. Simplest but by far the slowest is to ONLY import the images in Lightroom CC. They will sync up to the cloud and then download to Lightroom Classic. The advantage of this method is that it is simple. The disadvantage is that it is really slow as you have to upload the images to the cloud first and then download them (that's all automatic of course but still will take a while even on fast connections) before you can start using them in Classic. That is why method 1 is best.

P.S. all this could have been avoided if Adobe would simply add full raw sync capability to Classic. I started a thread here years ago on that (see link below). Please plus it even though this boat has long sailed unfortunately: https://feedback.photoshop.com/photoshop_family/topics/lightroom-classic-should-be-able-to-sync-full...

I suggest anyone interested should "sign up" at the above link John provided. They ask a few questions and then a closing message states you will be notified via email when it is launched. No actual launch date is given other than 2019.

Here: https://www.adobe.io/apis/creativecloud/ccstorageapi.html

"Recently, they've said it's coming in 2019. "

Where have you seen that, John?

As I described above, the Adobe Creative SDK inexplicably omits keywords from its APIs.  If it provided keywords (a very small amount of engineering), then plugins could easily sync keywords in both directions, even handling the common use-cases of hierarchical keywords. So add your vote to this feature request: https://feedback.photoshop.com/photoshop_family/topics/creative-sdk-provide-access-to-lightroom-cc-keywords

A couple years ago Adobe promised a shiny new CC Storage API that would provide access to all Creative Cloud "assets" using modern REST APIs.  Recently, they've said it's coming in 2019.  One can only hope that they don't omit keywords and other metadata from the API.   If they ever deliver a full-fledged API, then a third-party plugin could provide the full sync between Classic and CC that Adobe doesn't want to implement: keywords, smart collections, syncing originals rather than smart previews, more flexible deletion:
https://www.adobe.io/apis/creativecloud/ccstorageapi.html

Rewriting Lightroom is (was?) not a bad idea: LR is painstakingly slow and could use a facelift. Unfortunately, CC is not up to the task, like you said.

... and not to forget the « whole cloud » philosophy which is matter for debate. (Spoiler alert: never will I leave my pictures ONLY into the cloud nor will I support a company making countless efforts in keeping my data captive).