Does Quartus 18.0 support the VHDL 2008 'ELEMENT attribute?

Quartus has supported some basic VHDL 2008 since Q10 or 11 (I cant rememember, but before it was separated into Prime and Pro) but support has been limited to those listed on the page you linked to since then. So you can select 2008 because of this history. The page also clearly states the list is for Quartus Prime, and not Prime Pro.

With the intel transition everything has become rather confused. It is very difficult to find documentation about the standard Prime software, as everything appears to go via the Pro version. This is likely because all of the Prime software and supported devices were not part of the Intel/Altera merger, and intel are focusing on their new products, while neglecting the past.