The collection structure is simply something that is stored in the catalog. Nothing exists about the collection externally. If you want that collection as a separate entity you would probably do well to export it before you give up on Lightroom 6.
If you subscribe to the cloud-based Lightroom or Lightroom Classic which would allow you to continue using your catalog and file-based catalog on your computer, you would have the same option as far as Lightroom Classic is concerned. As far as Lightroom for the cloud is concerned there is an app available that will allow you to download everything that is in the cloud. I don't know a lot about it and don't know if it will download your images organized the way they are organized in the cloud. But you will be able to get all of your images to your computer in the event that you should decide to cancel your subscription.
If you stop a subscription to LR Classic the LR Library continues to function, so export is possible. Only the develop module stops functioning and Google (or third party maps) due to licencing arrangements.
If you stop a subscription to Lightroom (cloud-eco system) all originals stored in the cloud are available for two years. The Adobe downloader app also enables you to retrieve your original photos and edit settings.
A couple more comments from me-
While you have your Collections in Lr6- Use each Collection, by selecting these images, and add a keyword (eg. the Collection NAME). When you 'Save' metadata to all your files [Ctrl+S], they will then have Keywords to 'Collect' these images with future software. (Note: Raw files will have the XMP sidecar with metadata/keywords. If you want to convert your raw files to DNG then keywords will embed in the files and show in Mac Finder TBMK. They do show in Windows File Explorer-I know!)
As you are a long-time user of Lightroom, I might suggest that the Cloud Version (Lightroom v3) is not for you so do not even consider it. If you change your mind about the 'Subscription' choice then Lightroom-CLASSIC (v9.0 local folders etc) and Photoshop (in the Photography Plan 20Gb) would be the best choice. Any "Download from the Cloud" would then NEVER be a question, a problem, or need consideration.
Thank all of you for the answers. I like the idea of WobertC very much, the keywording by the catalog names. It involves a fair amount of manual work, but I think, the gain is worth the pain. Even if the "future software" will not be able to make use of these keywords, it will give you the possibility to organise the photos in any way you want by simple scripting (shell scripts, python, whatever).
Coming back to the decision: I am leaning towards continuing with Lightroom, it is such a powerful tool, and probably with Lightrom Classic and Photoshop as a bonus if I choose the Photography plan. However, I think it is essential to know what and how I can retreive if I cannot continue with the subscription for any reason.
It's certainly important to ask the exit strategy question.
Do remember that Adobe have confused everyone with the names. "Lightroom" is what they call the new cloud-based Lightroom which you should probably avoid, as Wobertc said. Lightroom Classic is the Lightroom you've been using, and you can use it in conjunction with mobile devices.
Also notice the important point about what happens after you end your subscription. Lightroom Classic will continue to work - you can't go into Develop or Map, but you can do anything else.
This sounds good. But are you sure that I can search, browse and open my raw files in LR Classic, and even make exports (with all previously made adjustments applied) after my subscription expires? For me it seems that Adobe does not allow you to use the functionality of Camera Raw if you are not a paying customer. For example the free Adobe Bridge opens raw files only if you have a live subscription.