One interesting followup. As it turns out, my installation of Snapz Pro X (2.5.1) was not actually compatible with Catalina. I assumed it was because it launched, but when I tried to actually capture something, I got an error.
Some web searching revealed that this is probably because the kernel extension was not signed. They signed it in the last version, 2.6.1, which I never downloaded.
I did see that many third-party download sites were offering it, but I won’t download from them, out of fear that they may contain malware (although I’m sure there are some sites that are reliable enough to use). On a hunch, I decided to visit an archived copy of Ambrosia’s page for Snapz Pro X (Snapz Pro X | Ambrosia Software, Inc.). There was a download link from there which, much to my surprise, downloaded an archived copy of the 2.6.1 installer.
The installer file was signed with Ambrosia’s certificate. Although that cert had expired in 2017, the Apple installer utility validated the installation package and it installed.
This version is still not 100% with Catalina. It quits (probably crashes) when trying to capture video, but it works great for all of its still-image capture capabilities.
Of course, if you don’t already have a license key, this download won’t help you, since the company went out of business about two years ago, so there’s nobody to generate keys anymore, but if you have a key for an old copy, you can still upgrade to the latest version.
I wrote all this in a blog post yesterday: Thank you, Internet Archive