Well… copyright law is a muddy area. Also, the law is not harmonized between countries.
In most cases, if someone designed graphics symbols that have traits of author’s individual approach then it’s considered Works and is automatically protected by the copyright law.
Now to the symbols. Generic symbols themselves are not protected by copyright because wel… they’re generic and fit for purpose. But if particular implemenetation of the common symbol is unique in some way for it’s viewer; then this uniqueness may be in the copyright area.
And even if the original shape is not “photocopied”, but the derivative carries these unique touches then it may violate copyright of the original author.
And the last thing any open source community wants, is to be engaged in lawsuits which are very resource consuming. So: if in doubt, avoid - is the most efficient strategy.
Kicad went through this with the old iconset which is being replaced with icons with traceable licensing scheme.
In case of Elektuur symbols: if they’re carrying individual touches, and their licensing is unknown, best to avoid them in KiCad official release.
And YES, I agree they make the schematic look much nicer than with plain generic boring symbols.