If I am correctly informed, the General Data Protection Regulation (GDPR) entitles any EU and EEA citizen to have right of access of any data stored about them (and non-EU/EEA citizens such access to data stored about them by organizations inside the EU/EEA). This clearly includes recommendation letters which in some form or another must be linked to their identity.
This probably changes the signaling value of recommendation letters so this should be of interest to both reference letter writers and applicants.
A side question would be whether this includes reference letters written before the GDPR went into force.