Upgrade late-2011 iMac as second monitor for a new M4 Mac mini?

Hi, I understand the latest OCLP + recent OS allowed to use old iMacs as monitors for other Macs - is this so? If so, what do I need to setup in the old iMac? A recent OS would be enough, then it would be recognized as screen by the host computer?

Can I do it through the HDMI output (I do have a cable for that) of my Mac mini?

(what a mess - I hope you understand my question)