You'll actually find that quite a lot of apps are still 32-bit and these may quit working after you update. I ran a check and many of the uninstaller apps supplied by software developers are still 32-bit. Older versions of Office apps. The player utilities supplied by AJA. Just a few examples.
Big loses for me will be MS Office program (2011 version), which I only use occasionally, but have a lot of old documents. I guess I will have to see how much I can use the simple mac apps of numbers, pages and keynote, sign up for the new MS Office or wait until I really need Catalina.