"deem illegal" is correct and usable in written English.
It can be used to describe the action of officially deciding or declaring that something is illegal or forbidden by law. For example: The Supreme Court deemed the new law to be illegal.
The job of a newspaper is not, as your mea culpa suggested, to act as a court, raising awareness only of what you deem illegal.
Since then, he has refused to drop any hint about the succession, presumably because he thinks that by doing so he would set off an ugly power struggle that could lead to his own speedy retirement.President Mubarak has never appointed a vice-president, which some constitutional lawyers deem illegal.
The individuals targeted by the sanctions are seen as having played a key role in the referendum, which Kiev, the US and EU deem illegal.
Kremlin-backed forces seized control of the Crimean peninsula, and the territory, which has a Russian-speaking majority, voted to join Russia in a referendum that Ukraine and the West deem illegal.
Twitter explains that it locks accounts if it suspects they've been compromised by hackers or violate "Twitter's Rules," which includes "unlawful use" that depends greatly on what national governments deem illegal.
Numerous countries censor online content they deem illegal, but cyber sovereignty takes on a new dimension in China, where global web giants such as Google, Facebook, YouTube, Twitter and Instagram are blocked.
The online review dispute, in contrast, was deemed illegal.
In this case it has been deemed illegal.
Under the cloud of a points penalty for a keel fin deemed illegal.
(Another crowdfunding firm, RocketHub, screens out only projects it deems illegal or in bad taste).
Unlike in the past, he said he now encounters situations "where my religion is deemed illegal".
