Installing Older Version of MacOS/OSX alongside current So with the end of 32-bit App support, I was thinking 
maybe if I run across an app that I really need to run than will no long run post 10.15, that it might be a good idea to have an older version on a small partition for this purpose. I was personally due to my likening of that version of OSX thinking about trying to install 10.11 on a separate partition on my Mac. However I keep hearing that it’s not possible to install an older version of MacOS/OSX (in dual boot) while a newer one is installed, is this the case?

My Mac will happily run 10.11 as it did back when it was current so that’s not the issue. It’s rather can I just get an installer ready and plop it on a blank partition and then just select it in the bootmanger with 10.15 and Windows, or will it not install because of having 10.15 installed currently?
Yes, you can certainly install two versions alongside.

The answer from benwiggy is incorrect in assuming that you cannot have APFS and HFS+ coexisting on the same physical drive. It is indeed possible, and very easily achievable using Disk Utility to create two partitions on the drive. One for the APFS container (which holds your APFS file systems), and one for the HFS+ file system.

Instead of running 10.11, I would recommended having 10.14 and 10.15 coexisting. The reason being that support for 10.11 ended a year ago - this means that if you want to connect it to a network, you'll probably end up with security related issues.