Out of curiosity I checked their site every week this year. The other day the search engine didn't show LR6. I see the the DVD is still available at B&H and I imagine on Amazon, etc.
Lightroom 6 has been discontinued and is no longer available for purchase on Adobe.com. Consider upgrading to the Creative Cloud Photography plan to get the latest updates in Lightroom Classic CC and Lightroom CC, and ensure that the software works with raw files from the newest cameras.
https://helpx.adobe.com/download-install/kb/lightroom-downloads.html
Lightroom 6 standalone application was discontinued in Dec 2017, when
Adobe announced that the last and final update LR 6.14 was released. They fully stated that there would be no further updates to this version, however they further indicated the application would continue to be available for users who did not wish to use the subscription application.
At that time the further upgrade to the Creative Cloud LR CC application (LR 7) was already available as a subscription only application, we are now at Lightroom Classic CC (LR (eight).
The way I see it is there was full disclosure.
I started using my first PC in 1985 with 5 1/2" floppies and progressed through multiple different media which are no longer supported or usable.