Yes, the European Commission said that adblock detection is illegal back in 2016. But they believe it's a legitimate interest, and said that they want to legalize adblock detection in a press statement back in 2017.

Can users still use ad blockers?

The proposal does not regulate the use of ad blockers. Users have the freedom to install software on their devices that disables the display of advertisement. At the same time, the Commission is aware that 'free' content on the internet is often funded by advertisement revenue. Important bit: Therefore, the proposal allows website providers to check if the end-user's device is able to receive their content, including advertisement, without obtaining the end-user's consent. If a website provider notes that not all content can be received by the end-user, it is up to the website provider to respond appropriately, for example by asking end-users if they use an ad-blocker and would be willing to switch it off for the respective website.

Edit: I highlighted where they state that they want to legalize adblock detection.