Several of the more powerful and IoT Arduino platforms currently run on top of MBed.
I'm not sure exactly how this (which I think mainly ends the MBed online site and removes ARM as a contributor to the open source MBed software) will affect the Arduino platforms.
To be fair, I have no idea how much ARM contributes to Mbed now.
IIRC, it started out as an NXP thing, and ARM got involved to propagate the idea to multiple vendors. (It was essentially NXP's answer to Arduino, back when it was more a collection of libraries than an OS.)