I have seen two questions about this asked in 2010 and 2011.
The answers there say that capitalised Internet is correct for the World Wide Web, and that lowercase internet is correct for any other similar network.
However, Google ngrams shows that usage of the capitalised Internet peaked in 2002 and declined until at least 2008 (by 27% by the time the data stop), while lowercase internet continued to gain in usage over the same period, although not overtaking the other.
I know that capitalised Internet is still prescribed, but does it reflect actual usage in 2018? Is lowercase internet now used more than capitalised Internet?