I remember seeing Myst V available as a digital download on sites like D2D, but I can't find it any more. Has anyone seen it anywhere?

I found this web page.

It's the official Myst V Ubisoft site and under the game info tab, there is a link: "click here to download the game".

If you click it, it does indeed navigate you to the direct2drive product page where one can read the following notice: "The current product you are looking for is currently not available."

Moreover, if you search for "Myst V" in d2d, you will find a "Myst V Guide" available.

So we can safely assume that the game was once available there, but their business deal must be over now.

Lastly, I cannot find it on any other digital distribution platform I know.