Since the link SlyCooper.com only redirected to a sub-domain of Sony's Playstation website, the site going offline does not mean that they failed to renew the web address; it most likely means that Sony intentionally unpublished the Sly Cooper website. Could this mean that they are abandoning Sly Cooper? Could this mean that they took the website down to perform updates and renovations on it to promote an upcoming game? Could this simply mean nothing at all? It is entirely possible that the site was taken down to free up server space. There wasn't much content on the official website, and it hadn't been updated since 2014 anyway. It does seem strange that Sucker Punch still own the website address SlyCooper.com. It's possible that they've always retained ownership since 2002. However, I remember looking up the website on WhoIs in the past and I thought I remembered seeing it under different ownership... but I have a terrible memory and could be mistaken. (SEE EDIT BELOW). Of course Sucker Punch announced just last week that they have been busy working on a new game, Ghost of Tsushima, which is a total departure from the Sly Cooper and inFamous series. This all probably means nothing, just figured it was worth reporting to see what you guys thought - let us know in the comments!
EDIT (3:45 pm EST): Upon digging deeper, I've discovered some other curious information. Both SlyCooper.com and SuckerPunch.com are registered to the same person: Michael Becker. According to his LinkedIn profile, Michael Becker has only been working as an IT Administrator at Sucker Punch since February 2015. So there HAVE been changes to the ownership/registration status of the website since Thieves in Time was released (though this could simply be because the domain was renewed - I dunno). I do wonder why Sucker Punch are the ones renewing and retaining ownership of the Sly Cooper domain, and not Sony (who, correct me if I'm wrong, presumably holds the IP for the Sly Cooper Series?). Of course any company wouldn't want an important domain - even for a retired product - to fall into somebody else's hands. It just confuses me as to why Sony isn't handling it since Sucker Punch is done with the series. This also seems weird because, once the animated series is released, won't the companies involved with that venture want to use SlyCooper.com for promotional purposes? I'll have to do more research to see who the website was registered to in the past, and if it ever changed hands from Sucker Punch Productions before this more recent update in registrant information.