Our British Content Provider The National Archives UK, in collaboration with the UK Intellectual Property Office, have produced new guidelines on the usage and digitisation of copyright material. The guidance suggests ways to register collections and works as "orphan work", as well as best practice to conduct research on possible rights holders.
The guidance also explores the copyright exceptions and other factors that may allow an archives to proceed with the digitisation and digital publication of work, as well as general information on re-using public-sector information.
The information refer to UK law and the UK archival sector, but they can be a good starting point for reflections in other countries, too.
You can find the guides here: https://www.nationalarchives.gov.uk/archives-sector/legislation/copyright/