I'd argue it's sort of… insanely clear, when clicking on the download page, that there is no version available.
https://obsproject.com/ clearly shows it is cross platform.
https://adium.im/about/ clearly shows that it is not.
Either way it didn't cost the website owner much to lead on some people not in their target audience.
This is like the B2B SaaS marketspace - it's almost taken for granted that your app integrates with Salesforce. People are surprised if it isn't.
However, anyone who runs, Microsoft CRM, SugarCRM, Netsuite, etc. - are all used to hearing "Sorry, we don't integrate with X". I'd say ubuntu falls into a similar category....