On its website it is established that the new version 1.9 requires 10.12, however in the readme it is mentioned 10.9, without specifying that this and others below 10.12 require an earlier version. It seems that the version of the disk image requires that 10.9 that you mention.
Furthermore, the application’s info.plist file also mentions 10.9 as a requirement. This makes apps like MacUpdate consider that the version also requires 10.9 and does not mention the change.
Both for users and for those program update services, it would be interesting if the information was consistent. Or at least that the readme file was clear about the requirements of the version to install.
One of the advantages that came to the program when I began to think about replacing DEVONthink was precisely its low requirements, which in my case are fundamental since I still use two macs with 10.11. And it seems to me very appreciated that they continued to offer support to those systems.
Now is the time to abandon them, but at least they continue to include information about the previous versions that do support them, something I appreciate. Many others consider all these versions to be disappeared in limbo, but they are still of interest to many users. And it is something totally necessary. For me, true ecology is demonstrated in these details. Even more basic than making new products with materials with an ecological sticker. The true ecology consists of extending to the maximum the useful life of the products. And that in computing happens to maintain software support for these products. Thank you.
P.S. Mechanically translated message as I do not speak English, I apologize if it is not understood or I have not expressed myself correctly.