DSSP is effectively a dead language. There are a few interpreters, but for a long time there weren't, so there hasn't been much code for decades now. It died with the U.S.S.R., like many innovations in computing did.

I wondered too (first I've heard of this) but all I find is in Cyrillic: https://github.com/trinarium/DSSP-C

I did find one short paper in English: http://www.complang.tuwien.ac.at/anton/euroforth/ef00/lyakin...

Here's an interpreter with docs in English:

https://github.com/beadleha/libreDSSP