A couple of others personally confirmed in the Discord that you can still play delisted games you've already bought.
It's not ENTIRELY without the realm of possibility that a delisted game also becomes impossible to download, like PT, but that seems to be the only example so far because 1) it was a freebie so nobody really owned the license in the first place and 2) Konami.
Steam offers the ability to back up game install files last I checked. Assuming the function was not removed due to fuckery, you should be able to create your own install discs (or disc image files) of delisted games for your own piece of mind.
EDIT:
Okay so the function does still work: its FAQ is
here. So go to your Steam Library, find the game in question. Right click and select 'Properties'. Go to 'Installed Files' and select 'Backup Game Files'. It will ask you to specify a directory so do that, and it will compress them down. Transformers Devastation went from about 9GB on disk to about 6GB. It doesn't include saves, or config fiiles though.
Now normally I'd say go download some CD/DVD/BluRay burning software - BurnAware Free is one example - and use its ISO making feature to turn those files into a disc image. Sadly after some testing, this process does not allow you to make an installer disc as from the days of yore, the ISO merely packages the backup files into a single file. So you can probably achieve the same thing by just leaving them in the folder and whacking them on an external drive.
This thread on Steam includes instructions on how to install from the backup files.