Is custom routing deprecated?

This blog entry has a warning at its top:

WARNING: This article contains outdated information. We no longer recommend taking its advice.

Does that mean the routing parameter should not be used at all anymore? Is there a reason? It seems beneficial to me to restrict a query to just one shard whenever possible.

No, it just means that it's nearly 3 years old so there are things in there that no longer apply.

However you can still do custom routing.