[PointyFox]

There's also the MDR-Z1R which is a closed back that almost sounds open.

 

[serman005]

ZMF Ori--semi-open.

 

[MattAsAHatter]

A perenially true statement...though recently there have been some headphones that don't fit neatly
into one category or the other.
A few worth naming - Sony MDR Z7 had some collosal vents - how many more would have qualified it as "semi-open?'
And maybe the answer arrived in the Z7MK2, with even more venting.
Not as well known, perhaps, but the "closed" Audioquest NightOwl has some sort of full venting design.

And on the other hand, Beyer refers to their DT1990's as "open".
https://global.beyerdynamic.com/dt-1990-pro.html

But hadn't wee seen that sort of design when it was called "semi-open"
e.g. the DT880's.

Just seems to me that the lines are beginning to blur.

Language is frustrating. Standards are convenient/nice to help language be precise in times of technical need (or otherwise) and a line should probably be drawn to divide open, semi-open, and closed. DBs of isolation or leak etc.