Is it necessary to reduce the feed rate when drilling slanted surfaces with the MULTI-MASTER exchangeable flat bottom drilling head?

Yes. When drilling slanted surfaces, the feed rate should be adjusted according to the angle of a surface inclination as recommended in the corresponding ISCAR guides. It can be roughly estimated that the feed reduction is 30-50% of a common value, depending on the angle of inclination.

Why does ISCAR continue to use outdated designations such as GGG for nodular cast iron when specifying engineering materials in different guides and ITA software?

The answer is very simple, outdated designations are still common in the industry and used by the manufacturer. Designations that begin with "GG" for gray cast iron, "GGG" for nodular cast iron (according to the old DIN standards), or "En" for steel (according to the old BS standards), have been replaced by other designations in their appropriate standards. However, despite the newer and formal changes, various outdated material designations are the everyday language of the professional world. Therefore, modern designations have been simultaneously preserved with a few outdated designations, which remain popular among manufacturing professionals.
As a side note, a similar situation may be observed with commercial names. Some materials are well known by their trademark and not by their standard designation.

Bird's nest, birds-nest chips

a clump of entangled metal swarf formed by long unbroken chips during the machining process.