I once did a lot of serials cataloging and including 'http' or 'https' was the standard we always followed. To double check I ran a quick search on a big pile of MARC records I had handy and found that, of ~120K records with an 856 with first indicator 4, only a very small percentage (about 0.2%) don't include the 'http' in a subfield u. And about two-thirds of those look to be cataloging blunders of one sort or another rather than deliberate decisions.
The records that aren't pure blunders all follow the pattern 'example.org', leaving off the leading slashes and trailing. If the http/https is variable or indeterminate for some reason other than the client's browser then you could just record it like that and add an explanatory note in a subfield z.
Center for Research Libraries
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Stuart A. Yeates
Sent: Monday, August 17, 2015 3:41 PM
To: [log in to unmask]
Subject: [CODE4LIB] Protocol-relative URLs in MARC
I'm in the middle of some work which includes touching the 856s in lots of MARC records pointing to websites we control. The websites are available on both https://example.org/ and http://example.org/
Can I put //example.org/ in the MARC or is this contrary to the standard?
Note that there is a separate question about whether various software systems support this, but that's entirely secondary to the question of the standard.
...let us be heard from red core to black sky