Hi MissMarcoPolo,

The updated ref tagging is based on Vegaskrá 2024, a public document in Icelandic published by the Icelandic Highway Administration (Vegagerðin), available for download from https://www.vegagerdin.is/samgongukerfid/vegakerfid/vegir .
According to OSM wiki ( https://wiki.openstreetmap.org/wiki/Iceland ) it's a public domain document that can be used as a source for highway names and refs.

The document is quite heavy and local knowledge is often needed to decipher the alignment of the roads. The data from Vegaskrá is also published as a GIS feature by LMÍ (National Land Survey of Iceland) under the IS 50 V set of GeoPackages for features in Iceland ( https://gatt.natt.is/geonetwork/srv/eng/catalog.search#/metadata/41D3314D-55AF-4FE3-9A9F-B9CB18AC5CAA ).
The IS 50 V geopackage from LMÍ is licenced under a revised version of the UK "Open Government Licence" ( https://www.lmi.is/is/moya/page/licence-for-national-land-survey-of-iceland-free-data ).
I have personally refrained from using the data directly as I am not sure whether data under this licence may be used on OSM.

Cheers.