For a while now, I've been using "(a)" notation to denote archived versions of linked pages. This is a small effort towards creating Long Content (a) – content that has a lifespan of decades or centuries, rather than months or years.
I think basically anyone whose writing includes links to other work should include archived links alongside the original hyperlinks, if the writing is intended to be long-lived. (And if you're not trying to write long-lived content, what are you doing, even?)
Practically, archive.today is my first choice for creating archives of webpages. It's free to use, and it's hard for content to be removed from the archive. (Folks can't just email in requesting that arbitrary content be removed.)
But archive.today can be slow to save new pages, and its library is fairly small.
archive.org is my second choice. It's run by the Internet Archive (fun aside (a)), is free to use, has a massive library, and is quick to add new pages. Unfortunately, folks can remove arbitrary content by emailing in, so I expect archive.org to be less durable than archive.today in the long run.
perma.cc also seems promising. I don't use it because it's expensive if you don't have an academic affiliation.