Is there a way to be able to run old 32-bit apps in updated macOS Catalina in an old 2012 13" MBP?

If you are on an Intel Mac, a hypervisor / virtual machine can allow you to install and boot an older macOS as a guest, and run apps within that. Parallels and Fusion are commonly mentioned hypervisor choices here. That written, you’ll want to be looking for app replacements or app updates for these older 32-bit apps, as the path you are on only gets more and more gnarly over time, and you’ll have some larger issues ahead with running older apps for Intel processors on newer Apple silicon Macs.

If you are determined to run Catalina, you could install it on an external drive and boot from it. Of course you would have to boot from the internal Mojave installation to run 32bit apps. As I mentioned, there's no real advantage to running Catalina over Mojave at this point.

antdude wrote:

Yeah, I saw. I was hoping without slow speeds. VMs are slow. :(

Emulators do have performance issues, at least until any translation caching gets warmed up. Guests in a hypervisor run at near native speeds as the vast majority of hardware instructions are simply executed directly. The guests do consume substantial system resources; memory and storage and processor commensurate with the scale of what is running, which means under-purchased systems will be slow. For cases where native boot is supported, full-reboot switching among operating systems is more time as compared with switching among the running system and any guests. Or you can work to remove these old apps, as you can be assured the work and the costs involved here will only increase. I work with folks that routinely seek to keep apps originally from the 1980s and 1990s working, too. It gets expensive. Rock. Hard place. Etc.

PS: if partitioning is involved in your eventual solution, and if re-partitioning is ever planned, have backups of everything. Corrupting partitioning arises, and the fix is re-partitioning and reloading the whole storage device; restoring all partitions from backups. Guests in a hypervisor avoid this particular mess, as the guests are big files that can be copied around or archived or deleted as needed.

FWIW, if you don't want to run older versions as a guest VM, you could always downgrade the MBP to Mojave which was the last version of macOS that supports 32bit apps.

antdude wrote:

How about multiple macOS versions like I do with BootCamp (32-bit macOS v10.14.6 and 64-bit Windows 10)?

Newer Macs won’t boot a macOS version older than the version that the Mac first shipped with.

As for the older apps, macOS Catalina and later don’t have 32-bit support.

Figure out how you can update or can replace the app, or can migrate its data, because this isn’t going to get any easier, and is likely to get more difficult. Particularly keeping the older hardware working, and also keeping the older systems networked with the rest of the world as it updates as apps are only very rarely operating in complete isolation with no connections and no data transfers with other servers.

Apple started the 32- to 64-bit transition with OS X 10.5 and 10.6; a long time ago.

antdude wrote:

It's actually already on Mojave v10.14.6. I was hoping to upgrade to Catalina and still use 32-bit apps with hacks, tricks, etc.

The only "hacks, tricks, etc." have already been covered.

Just to add, since macOS Catalina is no longer supported there's not much advantage to upgrading from Mojave.

Just to add, no virtualization software supports accelerated graphics for macOS/Mac OS X guests. So they tend to be considerably slower than Linux or Windows guests.