On October 7th, 2019, Apple released macOS 10.15 Catalina. MacEditionRadio suggests that readers refrain from upgrading until they are confident that their applications will not be affected.
The primary reason for this concern is that macOS 10.15 Catalina is the first version of macOS that does not support 32-bit applications. Since many existing applications are still 32-bit only, this lack of support will likely cause serious compatibility issues for many users.
Current versions of frequently-used applications as of October 7th, 2019 that are known to be 32-bit only or contain 32-bit elements include Adobe Creative Cloud (even the latest version has utility apps that are still 32-bit), Apple Aperture, DataComet-Secure, Fetch, Steam, and Symantec Endpoint Protection (the installer does not function).
Many older versions of commonly-used applications, including Microsoft Office 2011 (and previous), all versions of Adobe Creative Suite, and many older games do not function. MacWorld has published an article on how to find the 32-bit applications running on your system.
The move to 64-bit only applications—something that ends a process Apple began in 2003—will be a good thing for the macOS in the medium to long term. In the short term, it means that macOS users will have to be far more careful with an operating system upgrade than usual.