You didn't follow the instructions. Go to the OP on this thread and search for "Catalina"
I don't mean to hijack the topic, but I can report back that I got my AlesisFirewire.kext to load to /System/Library/Extensions. It's a pretty beautiful thing.
I'm now using my old 8-input Alesis MultiMix8FireWire mixer with a 2017 iMac running OS 10.15.2. I'm using the mixer to a Firewire400-to-Firewire800 cable plugged into a FireWire800-to-Thunderbolt2 adapter which is plugged into a Thunderbolt2-to-Thunderbolt3 adapter into the USBc port on my iMac.
I tested it out in GarageBand, and it works - multi-track recording at once and everything! Thanks, tonymacx86 community!
Steps I used to get it working:
1. Boot the iMac into recovery mode via [Cmd] [R].
2. Once at the recovery screen, go to Utilities>Terminal and type the following:
csrutil disable; csrutil status [enter]
3. Then type the following:
nvram boot-args="kext-dev-mode=1" [enter] and reboot.
4. Once rebooted, place the kext on the desktop and run Hackintool.app.
a. Select "Tools", then the first little kext icon at the bottom, and then follow the steps.
b. Verify that it shows up in /Library/Extensions. If it doesn't, then go to step c.
c. Use kextBeast and set it to /Library/Extensions. If it doesn't work, then go to step d.
d. Open Terminal and type the following:
sudo spctl --master-disable
sudo mount -uw
sudo killall Finder
sudo cp -R ~/Desktop/AlesisFirewire.kext /Library/Extensions
sudo chown -v -R root:wheel /System/Library/Extensions
sudo touch /System/Library/Extensions
sudo chmod -v -R 755 /Library/Extensions
sudo chown -v -R root:wheel /Library/Extensions
sudo touch /Library/Extensions
sudo kextcache -i /
e. Try kextBeast again, but try the /System/Library/Extensions folder as the target again.
f. If it still doesn't show up again, then run the normal "AlesisFirewire Driver [v3.5.6].dmg" driver installer from Alesis. It should magically work this time.
5. Reboot.
6. If it still doesn't work, then do step 4-5 again.
7. For kicks, run the permissions fixer.
8. Reboot back into recovery mode and go to Utilities>Terminal and type the following:
csrutil enablecsrutil status [enter]
nvram boot-args="kext-dev-mode=0" [enter]
reboot [enter]
9. Verify that the audio source shows up now in System/Library/Extensions, and as a sound input in System Preferences, and test it out in Garage Band with multiple sources at once.
I expect someone could shorten all of these steps into something super efficient. The steps in the initial post of this thread didn't work by themselves on an actual iMac. But, it's working now. Woot woot!