You can run Terraria on macOS Catalina 10.15. You don't have to do anything special.
Download the latest version of Steam (Sep 3, 2020 at the time of this writing).
Search for Terraria.
Steam will warn you that it won't work, but that is an outdated warning for a previous 32-bit version of Terraria. Ignore it. The most recent version works fine.
(Steam really should remove the warning.)
Buy and download the latest version of Terraria (1.4.0.5 at the time of this writing).
And that's it. Tested with Terraria 1.4.0.5 and macOS Catalina 10.15.7
It may only work in an admin account. But it does work.
Google will give you a jumble of different answers about whether it works but here is the screenshot to show that it does run.
Unable to run Steam as non-admin on macOS
Unlike Epic Games, I wasn't able to get Steam to run in a non-admin (standard) account. It gave me some weird untrue non sequitur damaged file error ("steam_osx" is damaged). If anyone has any info on running Steam on macOS 10.15 in a non-admin account, please let me know.
Comments