Let's look around
Here's a peculiar quote from Canonical's site:
Lower costs
Ubuntu Core is free. It can be distributed at no cost, with a custom kernel, BSP and suite of apps to suit your device.
Here's another one, that's disclamer from Ubuntu wiki:
Building and using a custom kernel will make it very difficult to get support for your system.
While it is a learning experience to compile your own kernel, you will not be allowed to file bugs on the custom-built kernel (if you do, they will be Rejected without further explanation).
Did you notice anything peculiar ? At no point do these quotes state that result of using custom kernel, your OS will be considered not-Ubuntu. It might void your warranty, but it's still Ubuntu.
What Ask Ubuntu help center says?
Questions that you may ask:
- Using and administering official Ubuntu flavors including:
- Running third-party applications on Ubuntu.
- Development on Ubuntu. Services provided by Ubuntu and Canonical. Any questions not mentioned below or here are great
- Services provided by Ubuntu and Canonical.
Use and administration of an Ubuntu system occasionally involves dealing with the kernel (shocker, I know). It is important for businesses and developers to sometimes have custom compiled kernel for better performance or security. This is a tweak, not distro alteration as far as I see it, same as if a user were to tweak their desktop.
What if we have a user who is data scientist, who has custom-compiled version of Python for performance ? That's still Python, it's just optimized for user's needs. And we still recognize such questions as on-topic, even though Python is very much integral to an Ubuntu system. Use of custom compiled version of desktop, or scripting language, or bash shell ( or any other shell for that matter ) doesn't alter the distro, and generally is accepted. I see no reason to ostracize questions with custom kernels.
Development on Ubuntu also involves kernels (another shocker). Our community also has kernel and drivers developers. And in the process of development they alter the kernel thus making it custom, but their sum total of all tools still adds up to the OS being Ubuntu.
Here's also a far fetched but reasonable question to ask: Ubuntu has Livepatch service (which would fall under "Services provided by Ubuntu and Canonical"), but doesn't a user alter the kernel thus making it custom when they patch it?
The point is that having a tuned kernel and having issue with it doesn't constitute question being off-topic. An OS is sum total of all tools, even if one of them breaks.