The project, with source and documentation, is here: dectalk/dectalk: Modern builds for the 90s/00s DECtalk text-to-speech application.. There's also an Emscripten version that runs in the browser: DECtalk for Web
There are a couple of problems, though:
64-bit only: it fails to compile under 32-bit Raspberry Pi OS
- the licence: the source became available in an unusual way, and the current owner of the IP appears to be bankrupt and is unresponsive to contact attempts. The former (commercial) lead developer passed away a few years ago, but before he did he uploaded all of the project source to the public DECtalk mailing list.