You could also virtualise your Mac, and keep your unupgradeable software on that. Boot into to run everything else, and boot your Mojave disk image in something like Virtual Box when you need to use the software that can’t get past the upgrade.
The other option would, of course, to keep an older machine in service to that end too. I’ve a machine on standby running 10.7.5 just in case. In some instances, I have needed it.
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