[twunde]

As others have mentioned design docs/BRDs, etc generally fell by the wayside with the rise of agile, especially in smaller companies and startups (colleagues from those environments may never have seen a real design document and possibly never had to document anything). However, over the last few years, I've seen many companies moving towards at least light-weight design documentation typically using ADRs https://adr.github.io/ and/or RFCs (https://github.com/rust-lang/rfcs). This is often because without these types of documents, its harder to grow teams as new members must either be taught be a senior member or learn from the codebase, both of which are slow.

As a secondary note, regulated companies may have architecture/network diagrams and disaster recovery plans but these may be located with the compliance documents and not in the general document store that engineers use.