The deprecation of FTP in browsers and its likely effects on search engines
One of the things going on in web browsers over time is that they're
in the process of removing support for FTP, for instance Firefox
once planned to do it this summer
and Chrome may already have removed it. The obvious
reason cited by Mozilla and Google for this is that use of
URLs is very uncommon in web browsers (and on the web), and the FTP
client implementation is a bunch of old code that must be carried
around just for this. Another reason is probably that the web as a
whole is increasingly moving to encrypted communications, and even
if FTP theoretically supports a TLS enabled version called FTPS, in practice only a vanishingly
small number of FTP sites actually support it.
As a sysadmin and someone who periodically goes digging for old documentation, I have some feelings and worries about this. The direct issue is that browsers are often one of the friendliest interfaces for digging through FTP sites; they offer convenient forward and backward navigation, visual display, and even multiple tabs (or windows). Terminal FTP clients (the general state of the art on Unix) are nowhere near as nice. However, this is the smaller of my concerns.
My larger concern is the issue of finding FTP sites, or finding that a FTP site has documentation I want. Generally I don't go to a FTP site and start hunting through it; instead, I do an Internet search and discover that some ancient thing on an old FTP site is the only source of what I want. Succeeding in these searches relies on the Internet search engines crawling and indexing FTP sites.
The major use of Internet search engines comes from browsers, and search engines are highly motivated to display only results that the browsers can actually use. If a browser can't use FTP URLs, a search engine has a reason to at least lower the priority of those URLs and may want to remove them entirely. As FTP URLs become lower and lower priority and get displayed less and less in results, search engines have less and less reasons to crawl them at all. And at the end of this process, I can no longer find old documentation on old FTP sites through web searches.
(As FTP sites stop being indexed, accessed, or usable in browsers, people also start running out of reasons to keep them operating. Many of the most valuable FTP sites for me are ones that are historical relics, and apparently survive primarily on benign neglect. Their contents are highly unlikely to be moved to HTTP sites; instead it's more likely that the contents will be discarded entirely.)
I don't expect this to happen imminently. It will probably take years before all of the infrastructure is turned off by some of the players, based on past experience. But I wouldn't be surprised if it's hard to do searches that return FTP URLs within five years, if not sooner.