In some older Boston Whaler boats I have observed that metallic fuel line was used to bring the fuel from the below-deck fuel tank up above-deck level. I assume that Nickel-Copper tubing was used. I also assume that flare-fittings were used. This practice seems to have been abandoned in favor of all-rubber fuel lines. That probably is a result of easier installation and routing of flexible rubber hoses instead of having to carefully bend metallic lines to a particular shape.
In regard to permeation of the fuel line--a subject of recent EPA concern--a metallic fuel line would be very compliant.
In the newer rubber fuel hoses their rating is specified as A1-15. The dash-15 nomenclature indicates the material complies with a maximum vapor permeation limit of 15 grams per square meter in a 24-hour period. The previous limit was for maximum vapor permeation of 100 grams per 24 hour period.
The federal regulations about permeation of fuel lines are given at 40 CFR 1060.12:
https://www.law.cornell.edu/cfr/text/40/1060.102A very comprehensive document about fuel system is
United States Coast Guard and Environmental Protection Agency
Compliance Guideline--How to Comply with the Law
Fuel and Emissionshttps://www.uscgboating.org/assets/1/AssetManager/ABYC.1002.01.pdf