Apple has finally fixed an inconsistency in the Mac App Store on macOS Catalina. As noted on the AAPL Ch. Site, the store no longer presents 32-bit applications, which are incompatible with this version of macOS. These old apps no longer appear in search results.
Thus, the risk of buying an app incompatible with Catalina is now zero, which was not the case for several weeks after the release of the new system.
Old 32-bit applications are still listed on the Mac App Store of old macOS as well as on the web. Besides, you can always open the product sheet of a 32-bit app (for example Rulers) in the Catalina Mac App Store via its web page, but you will no longer be able to download the software.