Unfortunately it is not. If you contact Datawatch you will get the standard line "We only support the last two versions". I am in the same boat as you, I have physical copies of 10.0, but 10.5 was a download only. I even paid for extended download service, but that portion of Digital River has been sold off and the new company has no record of me.
Last month I paid over $4000 to Datawatch for some new licenses in hope they might give me a 10.5 download link and they still refused to allow me to download 10.5. It still irritates me greatly as I have been under maintenance since they started offering maintenance with V11.
Wes, thanks for the response. I remember downloading the V10.5 upgrade very soon after I purchased V10.0. The thing I can't remember is whether or not Datawatch sent me a key code for the V10.5 upgrade or if I just installed it right over V10.0. I can't find a key code for V10.5 and I'm usually pretty good about keeping those things.
I chatted with a rep and of course got the same answer as you did. The have nothing available below V11.0. Then he went into his sales pitch about V12.0. I was happy with V10.5 and now I can't find it. But I'm still looking. The key code is problematic. I don't know if I ever had one for V10.5.