MacOS has been fully 64 bit since the release of 10.7 (Lion) in 2011.
Yet is has continued to support running 32 bit apps up until the release of 10.15 (Catalina) which removes support for 32 bit apps entirely.
There are ways to work around this like running an old version of macOS in a VM.
There are many apps that have not been updated for 64 bit. Things like AccountEdge (what used to be MYOB) wont be updated to be 64 bit. Old versions of MS Office need to be updated to newer ones as they too are 32 bit and will not operate on Catalina.
And anyone producing apps in Xojo should be building ONLY for 64 bit macOS. I’d encourage that even if you have to support old versions of macOS since all versions that Xojo supports are all 64 bit.
If you’re still building for 32 bit macOS you should update now and be building for 64 bit.
I’d encourage you to do the same for Linux as most modern distros are 64 bit.
The only target where you might need to still build a 32 bit and 64 bit version is Windows where you can get both 32 and 64 bit versions. While most machines sold today are 64 bit and installing a 32 bit OS on them doesn’t make much sense there may be legacy machines running a 32 bit version.