Please don't editorialise article submission titles. Even from the bullet point which you cherry picked to use as the title doesn't tell us anything useful other than it's a 271,800 character symbol.
I know it's annoying, even the article doesn't reveal to us what it is:
The longest demangled ELF symbol is 271,800 characters and is defined in the file usr/lib/x86_64-linux-gnu/libmshr.so.2019.2.0.dev0 provided by the libmshr2019.2 package.
Sadly this submission is a victim of the wrong kind of title editorialising.
After unpacking the data.tar.xz from the deb archive, I unpacked libmshr.so.2019.2.0.dev0 from the specified path.
Doing this:
readelf -W --demangle=c++ -s libmshr.so.2019.2.0.dev0 | awk ' { if ( length > x ) { x = length; y = $0 } }END{ print y }' > test.txt
gave me a text file that is 101,943 bytes in size. Basically a huge C++ template soup, but still quite a bit short from the 271,800 bytes in the headline.
If you want to see it, I compressed (568 bytes!) and base64 encoded it:
https://news.ycombinator.com/newsguidelines.html