Is the late 2015 iMac 27inch Retina 5k compatibility with the M2

Target Display Mode is dead tech to which Apple delivered the coup de grâce in early 2020. Its very restrictive hardware and OS requirements are outlined in this Apple article:

Use your iMac as a display with target display mode - Apple Support

Both your devices are too new.

I cannot in good conscience recommend any of the third-party work-arounds, especially for a primary monitor on a Mini. Almost all are laggy That might be tolerable on a secondary montitor for static items like pallets or referece documents., but would be awful for the primary. One of those developers goes as far as to include this statement on thei support home page:

Their red emphasis, not mine

A decent free-standing monitor costs little more that some of the third-party solutions and will proved full video options, unlike the iMac.

If you mean if the iMac can act as display for the mini then no it can’t act as display for anything without third party software which mean the mini would need another real monitor

https://9to5mac.com/2024/02/02/use-imac-as-monitor/

-KD- wrote:

Ok, that’s what I thought. A guy working in the Apple device section on my military installation told me repeatedly that I absolutely could.

Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014. It never returned. No Retina iMac – including yours – supports it.

In addition, Apple retroactively added restrictions on the video source for older iMacs that do support some form of TDM. Now the video source has to be a Mac released in 2019 or earlier, running Catalina or earlier. A M2 Mac mini fails both tests and therefore does not support using ANY iMac as a Target Display.

-KD- wrote:

Ok, that’s what I thought. A guy working in the Apple device section on my military installation told me repeatedly that I absolutely could.

Not surprising. When COVID forced the work-from home scenario on the world in 2020, web "journalists," starved for funds and devoid of editorials support and research skills, started posting flight-of-fancy articles on the magic of TDM. That was about the time Apple removed the last bit of hardware support for it. Remarkable but bogus claims abounded, from "it works on any device out there" to the ridiculous. One would think TDM could part the Red Sea from the wild claims out there.

It seemed people opted to believe non-Apple sources.

Before COVID, we saw maybe1-2 inquiries per month in the iMac forums. Suddenly we were getting a dozen a day. Most started, "I read this online article..." or "FaceBook says..."