Any suggestions for a Linux distro for a late 2015 27" Intel i5 iMac?

I would recommend Linux Mint as a starting point. A co-worker has installed Linux Mint on many of our organization's older Macs without any problems.

The default Ubuntu tends to require a lot of system resources to run the Gnome desktop. Plus Ubuntu has been changing things that some don't like & think it may cause problems. I gave up on Ubuntu decades ago due to all of the quirky behavior since I used alternate Desktop Environments which didn't always receive the same level of attention. If you decide to go with Ubuntu, then try one of the other Ubuntu flavors such as Kubuntu .

Linux Mint is a customized version of Ubuntu, but using a different default Desktop Environment and has omitted some of the questionable Ubuntu changes. Linux Mint has also tried to simplify the Linux experience (I'm mixed on this point, but it is a nice starter distribution these days).

I highly recommend you read up on some of the Linux Mint documentation to understand some basics especially related to software management & installation. Same goes for Ubuntu if you choose it. I don't know what either distribution's documentation is like since it has been years since I looked at them. I know the Debian Administrator's Handbook guide is a very good source of information for Debian based distributions (especially to understand package management & installation of software -- Chapt 5 & 6, some of it will be very in depth, the basics are fine) since much of it applies to Ubuntu & Linux Mint except where those other distributions have made significant modifications.

While most any Linux distribution should work, some may require some manual intervention & customization by the user since some distributions may have boot issues due to how they install the bootloader because the Mac firmware is even more basic than may PC UEFI implementations. Linux Mint & the Ubuntu flavors should install & boot a Mac without much trouble. Other Linux USB installers may boot fine, but their full Linux installation may not boot due to those issues at least without some intervention.

Linux will run just fine with as little as 4GB of memory for basic tasks, but for some other more involved tasks & workloads more than 4GB would be beneficial. 8GB of memory should be sufficient for most tasks especially if macOS was working fine with 8GB.

FYI, I highly recommend you first create a bootable macOS USB installer now while you still can so you can easily reinstall macOS if you ever decide Linux is not for you. I would suggest making sure the macOS USB installer will actually boot as well. This just gives you an extra option just in case you have issues with Internet Recovery Mode.

Create a bootable installer for macOS - Apple Support

Probably Ubuntu or Mint would be the better choice with less learning curve.

Note it won't be as straightforward or seamless as macOS is in either case.

Also, its to going to be anymore supported than an "unsupported" version of macOS.

You are just changing one unsupported OS for another different yet equally unsupported OS.

Fortunately, you can burn an installer disc for either version, and try it from the disc before actually installing it so you can see how it works before committing.

Another vote for Linux Mint.

Ubuntu 24.10 with the added Cinnamon Desktop. Preferably installed in a Virtual Machine and appropriate Mac resources. This is what I run in Parallels Desktop Pro v20.3.1 on my Core i7 2020 iMac.