Do definitions in headers still help optimization?

[Licht Martin Werner via Gcc-help <gcc-help@xxxxxxxxxxx>]

Dear all,

Does keeping definitions of functions in header files still help the 
compiler optimize your code? Or is that advice outdated?

The answer used to be a certain yes when all optimizations were 
exclusive to the translation unit. By now, link-time optimization has 
been a thing for a while, and one might imagine that defining functions 
in header does not make a difference for optimization anymore.

(Of course, placing functions in header files or asking for link-time 
optimization will affect the compilation speed and memory use)

Discussions of this question seem to be based on hearsay, so I'd rather 
hope for an answer straight from the source. Thanks.

Martin