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

Once again, you’ve proved it’s not a technical issue but a marketing decision.

It is really easy to understand.
Adobe does not want you to manage your keyword. As far your use Classic, you are in control. As far you use mobile, you lose control on your asset. Everything is done you can't use Classic and mobile in conjunction to make your asset virtually "captive". 
So Adobe will not make it. You have the choice of using the old and slow Classic or the new and fast but weak mobile.
Pay may friend, pay.

Even keeping the constraint that LR Classic uses hierarchical KW's and LR Cloud uses a linear (flat) KW structure, this problem can be solved.  There are, of course, some technical aspects but the main rationale I've heard so far for not doing this is the logical problem of how to map a hierarchical structure to a linear structure.  One solution is to convert LR Cloud to a hierarchical structure but putting that aside, here is one possible solution to that problem:

1)  In LR Classic, Adobe can add check boxes in the create/edit keyword dialog for

• "Sync this KW with LR Cloud"
• "Sync Containing KW's with LR Cloud"
• "Sync Synonyms with LR Cloud (would become regular KW's in LR Cloud)

2)  When a user ticks any of these boxes, LR Classic would check to assure that it is not redundant.  For example.  If you already ticked KW "Red" under "Flowers", and now you are trying to tick keyword "Red" under "Cars": it would reject the request as a duplicate.  By doing this, it will enforce a one to one correspondence between ticked KW's in LR Classic and KW's in LR Cloud - thus allowing two way sync

3)  If an image in LR Classic gets or loses a keyword with one or more of these ticked boxes, or the tick's get changed, that change can be placed in the Smart Preview (or XMP data) and can sync to LR Cloud.  This may include adding the KW to LR Cloud, adding an LR Cloud KW to the image or removing a KW from the image in LR Cloud

4)  If a user defined KW is added to an image in LR Cloud it could map to either no KW's in LR Classic or at the most to one "ticked" KW in LR Classic.  In the former case, LR Classic would create the new KW at the root level of the Hierarchy with "Sync this KW with LR Cloud" ticked.  In the later case there would be only 1 ticked LR Classic Keyword with that name that would be affected.

THIS CAN BE DONE if there is a will to do so at Adobe and the more people who "vote" for having KW's sync between the ecosystems, the more it will push Adobe to re-consider this.   

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