Is the hard disk of a late 2015 27" iMac 5k still replaceable?
Yes - Apple provides an internal SATA drive with standard connectors but custom firmware.

According to iFixit if you take the screen off carefully you can replace it, you may have to buy a bracket to secure it SSD that don't match the existing 2.5 inch drive mounting screws. 

Looking at the steps it doesn't seem to be to complicated to do but with the 5k iMac you need to be extremely careful since you have to remove the speakers. 

I would paste in the step by step images however there a load of them and you would be better off going to the site and taking a look at the images.

Also make sure you have the right screw drivers since its a t20 screw you would need a T20 * 100m screw driver.