C11 will run (of sorts) under HighSierra (10.13)…but you’re going to experience graphical issues…most notably how plug-in GUIs (both 3rd party and Steinberg’s) are displayed. This usually manifests itself with completely un-editable solid black or white GUI displays or GUI menu scrolling that doesn’t function properly as you attempt to scroll. In some cases, you may be able to work around the problem by using the plug-in’s Generic display option…or use “tear-off” menus in some of the Steinberg titles, if the feature is present. Worst case scenario is that merely launching some plug-ins will cause Cubase to crash. It aint pretty.
All of this is occurring because your iMac doesn’t have a Metal-compliant GPU. That’s why Apple won’t let your iMac download Mojave…and why Cubase 11 won’t run on High Sierra properly: C11’s graphic engine requires a Metal-compliant GPU to run as designed. Unfortunately, older iMacs and MacBooks unequipped with Metal-compliant GPUs are impossible (or near impossible) or non-cost-effective to upgrade.
(Many older MacPro towers (2010 and later) can be retro-fitted with Metal-compliant GPU’s, and can run 10.14 or 10.15 and C11…so if you’re reading this and fall into this category, there are solutions, as long as you throw money at it).
So…you can take your chances attempting to run C11 on your iMac and High Sierra…but the end result will be by no means, optimal and pretty much not may not be worth the effort. It’ll be up to you to determine that or not. My advice? Stick with C10.5 until you can (or if) you get yourself a more recent Metal-compliant Mac, if you wish to continue going forward on the OSX platform and want a relatively trouble-free experience.