Hello,
DxO is a relatively small company with limited resources. Generally speaking, supporting more OS versions means more testing work and more complex code to write (because OS updates provide newer technologies that simplify our work, or fix bugs which doesn’t force us to write workarounds). So we have to make a choice between the amount of features you’ll be able to get, and the supported OS. We cannot just provide as many features AND support older macOS versions.
To give a more concrete example, DeepPRIME on macOS uses a technology called Core ML. This is what runs our DeepPRIME algorithms either on GPU or on CPU. Machine Learning is a rapidly evolving area, as such Core ML already existed in macOS 10.13 but was a lot more limited than it is in macOS 10.14. As of today, DeepPRIME cannot work on macOS 10.13 without writing workarounds for these limitations. And we come back to the choice between OS support and features.
I understand that this is frustrating and that major OS updates come with downsides like sometimes forcing you to update hardware or with software features that you don’t want. In an ideal world we would have unlimited resources and would write all the technologies ourselves which would make us independent from OS updates, but that’s not how it is. I just wanted to make clear that DxO doesn’t drop older OS versions just for stupid reasons.