There's no doubt
paywall should be added to our word list. It's in several dictionaries and is usually written as one word.
The earliest use of the word known to the OED was in 2004. They also have a 2006 citation from the
Guardian which has the word in quotes, suggesting it was likely to be a novel term to a lot of readers:
The New York Times site is vast (though it has shut off some of its content behind a ‘paywall’).
The word is presumably modeled after
firewall.
Should the word be classed as common? Paywalls are quite common online these days, but the websites erecting them don't generally use the word
paywall. The word is more likely to be used in articles about online publications. I wouldn't say the word is precisely a specialist IT term, more a media/business term. But in any case, I'm not sure the word is widely enough used to warrant common status at this stage. I'll install it as a rare word.