Using a 2015 (Retina 5k) IMac as a 2nd monitor with a 2023 (M3) IMac?

That Mac does not support – and never supported – Target Display Mode or being an AirPlay Receiver for AirPlay to Mac. (AirPlay to Mac is a lower-quality method of using a Mac's screen as a display.)

There is a third-party workaround called Luna Display, Even the Astropad site says that it has limitations, and is not advised for applications involving high motion content like gaming and video editing.

No. You can't. Just buy a monitor.

Target Display mode was extremely limited in the machines and macOS versions it was compatible with. It was deprecated years ago.

+1 nope the new Mac is too new and the 2015 is too old. Please refer to Target Disk Mode and that article will explain.

As mentioned, if you want/need an external display for the 2023 iMac you need to purchase one. Don't worry, they start at less than $100.

Using an iMac as a monitor - Apple Community