Things have changed. I’ve carried a penknife, with a blade under 6cm, on flights regularly since 2015. Airport security always measure it and give it back to me.
However, it is worth checking that if you’re returning from somewhere outside the EU, such blades are acceptable to that airport’s security.
I flew BA to Amsterdam, then KLM to Accra a few years ago and nobody was bothered by my penknife. On the return from Accra, a big soldier with a gun said I couldn’t take it, despite the airlines themselves allowing it, so I lost it.
Seriously? Wow!
I've inadvertently flown with one of those Swiss Key thing that @BrumJim mentioned on my house keyring but just because I forgot and it obviously looked like a key on the xray machine but I've seen folks have tiny nail scissors confiscated so seems weird they let you fly with a penknife however small. Every airport I've been there are posters saying you're not allowed any pointy or bladed articles in the cabin. I've just made sure I put them in hold case.