@elinejosefineh - At this point, there hasn't been any official information on whether EA will make SimCity 64-bit, or just keep it as a 32-bit legacy game. Aspyr has stated they will upgrade their App Store version of SimCity Complete Edition to 64-bit, but as of this writing it is not yet available. The App Store version is single-player, offline use only, so if you were playing on EA's servers, your regions and cities would no longer be available.
EDIT: As of Dec 21, 2019 Aspyr has listed their SimCity:Complete Edition version 1.0.3 as being 64-bit. Be aware that EA does not support the Mac App Store version. The App Store version is single player only. There is no access to multiplayer or any regions on the EA servers.
There are a couple of other workarounds you could consider: You can create a bootable external drive running Mojave, install a virtual machine with Mojave, or boot camp your drive and run your existing copy on Windows (if you bought from EA, you're able to run either Mac or PC versions of SimCity). There are detailed instructions on how to accomplish these, and more information on BluebellFlora's blog post Running 32-bit apps after macOS Mojave
That's all the information I have right now. You may want to subscribe to the post [Mac] SimCity and MacOS Catalina I'll try to update it with any additional information, if and when available.
