Any ETA on when this gets updated to 64-bit on mac?

Interesting question. On steam there is a red box simply saying "This product is not compatible with macOS 10.15 Catalina.", so the devs are aware of this issue but most likely don't have an ETA when they will update. The problem is that Shadow Tactics is based on Unity 5.4.4 (released in January 2017), which is 32bit only on macOS, while on windows it already has a 64bit version. I also checked 2 unity based applications i have here from university projects: the one built with Unity 2017.2 (released in October 2017) is 32bit, the one built with Unity 2018.2 (released in July 2018) is 64bit, so I guess that Unity moved to 64bit on macOS with 2018.1 (released in May 2018). So getting Shadow Tactics running on Catalina, either requires to bring the currently used Unity version to 64bit (which would most likely require more than just recompiling the code, because in that case Unity would have moved to 64bit earlier), or updating to a unity-version that is 64bit (which would most likely require a lot of work because there have been changes to the engine so most likely some things will no longer work.

Just bought the game from my phone after seeing it had an Apple logo in supported platforms (no Catalina warning). Downloaded it once I got back to my keyboard. Only upon trying to run it did steam tell me it couldn't...big fat bummer

It's utterly insane that someone would release a 32-bit game in 2016, let alone leave it 32-bit in 2020. It's inexcusable, really.

Well, when they get around to recompiling and fixing this for 64-bit targets, I will likely buy it. I could play it now with a compatibility layer, but honestly, why bother? There's plenty of 64-bit games I could play instead.

As I wrote before, the problem is not the game itself (which is written in C#) but the underlying engine (Unity), that was 32bit only on macOS until early 2018 (which was very surprising to me, when I found that out). Additionally the main target platform for games is usually windows and maybe consoles, so unless the devs use their own engine or the macOS (and usually Linux) version is made by a company for which porting games to macOS is a main job (and by that is aware of those problems), the devs use windows for developing the game and probably only do some tests on macOS. And that is usually enough, since nobody would expect that a game engine, that offers a 64bit version on windows, is 32bit only an OS that supports 32bit software but that is mostly 64bit (and that quickly dropped support for PowerPc processors a few years back).

So yea, it would be cool if they release a 64bit version of the game, but the question is: would that pay off? Because they would either have to update the engine on their own to 64bit (which requires access to the source-code, which they don't necessarily have, the corresponding development tools and knowledge about these tools), or they would have to update the game to a newer version of the engine (which would require a lot of work since there have definitely been some changes in the meantime and most likely cause a lot of bugs). And besides that, they would have to do that for all plattforms, since they can't just update one platform to another version of engine, because that would make fixing bugs extremely difficult.

By the way: since the last post here the devs (Mimimi Games) announced a standalone expansion called Aiko's Choice, which will be PC exclusive (initially only Windows, but 32bit Linux is planned), And as they explained in a post called "Aiko’s Choice will be PC exclusive!" (sadly I can't post the link for some reason) on their website, releasing Aiko's Choice on all platforms would require a lot of additional work (which costs more time than they as a single project focused team can spare), because for consoles (and also 64 bit macOS) they would have to update the engine, which will require a lot of changes to the code.

The same also applies to a 64bit version of Shadow Tactics on macOS, which is why the devs won't update Shadow Tactics on macOS to 64bit.