will photos created in Lightroom forever be available once exported

With most cloud services, a file is not really stored in only one place. It looks like it is, but typically a cloud service will automatically back up the file to multiple geographically distributed servers. So if something bad happened to one data center with your files on it, another data center would automatically take over and you should never notice that there was a problem. In this way, in theory your files should technically always be safe as long as the subscription is active. 

If one day you wanted to stop your subscription, Adobe provides the free Lightroom Downloader so that instead of having to manually export your files, you just tell Lightroom Downloader to download everything from your account to your own computer. The last time I checked, you have an entire year to do this after you stop a subscription. 

do they mean one these are exported in their own right onto hard drives/folders on the computer/phones those photos would no longer be able to be opened either?! 

No, that would never be true. When you export a file to your own local storage, in a standard non-proprietary file format such as TIFF or JPEG, you have full control over it and you should be able to open it any time you want, in any application that can open that format, from any company. 

When we export images from the Adobe cloud to local devices (desktop computer, phone, tablet, social media etc.) we are creating a derivative copy of the image, the original remains in the cloud, and remains accessible.

Regarding the question if whether photos will “forever be available”, that would depend on whether we continue to pay our subscription fees. If we decide to stop our subscription we will have up to 12 months to download a copy of those images from the cloud, after that 12 months they will be deleted.

If something was to go wrong with Lightroom, it’s difficult to imagine all the different apps (Lightroom Desktop, Lightroom Mobile, Lightroom Web) being affected at the same time in such as way as to prevent us accessing our photos. I suppose we should “never say never”, but personally that scenario is not one that I lose any sleep over. There are some things that we could do to deal with that doomsday scenario, such as using the Lightroom Desktop option to store a copy of all photos locally on a hard drive but that only stores the original files, it does not include any edits. An alternative would be to export a copy of all cloud images (either as Jpeg/Tiff, or original + settings) to local drives. FWIW I don’t do either of those things.