We have been testing on developer betas of Sequoia, and we will provide an official statement about compatibility when (or shortly after) Sequoia is released next week.
As Ben says, Dorico’s existing Quick Look generator will no longer work in Sequoia because the old deprecated framework has now been removed. We already have a new Quick Look app extension in Dorico for iPad, which we can port to macOS, but there are some complications with how app extensions are provisioned. We’re not yet sure whether we will introduce a Dorico 5 update that includes these app extensions, or whether this will wait for the next major version. (It depends how complicated it is to retrofit the new stuff to the v5 code.)
The other thing to be aware of is that there is a new permissions prompt (well, there are many new permissions prompts, but one of particular relevance) that will appear when you run Dorico for the first time about allowing access to the local network. It may appear multiple times (for e.g. the MediaBay server, and for the audio engine), and you have to click Allow on each one. If you don’t allow this access, the audio engine won’t start correctly, and consequently Dorico will also not start correctly.
We will provide full details via our support pages in due course.