So far I have only come across this page for Samsung Smart TV and it lists the video streaming app that I'm after (MUBI) which I am reasonably confident is indeed available on Samsung TVs. So that I suppose gives some credibility to the correctness of that list.
Other than that I think the only way to figure out what is on a TV set is to check what platform is used by the vendor from this Wikipedia page, then check somewhere what platform is used on a particular model from that vendor as the maker may be using multiple platforms, and then based on that somehow infer what might be available.
Say, if the telly runs on Android TV, then presumably you can install anything you please from the Play store. I'm not sure, I'm new to this whole smart TV business, so someone else may need to confirm.
Some websites selling TVs, like AO, list apps, in the case of AO it's under 'Smart Services' section, but I don't think these are exhaustive lists.
Another option is to forget about this whole question and get a Chromecast or something like that which will work with any TV set. This is my plan B, but I'd rather have all I need available from the TV already. I don't want dozens of other gadgets, cables etc. to provide what's missing from the TV.