Be sure to see if you are using any plugins that are not 6.11 compatible.
First up is NerdPack (provides ability to install more smaller things), if so transition to NerdTools (similiar to NP but 6.11 compatible) or plan to after the upgrade. I think the approach I used was to write down what I actually used from NP, uninstalled it, upgraded OS, installed NT, then installed what I used from my list.
Next up is NUT which is an alternative Battery UPS plugin. Most simply use the normal APCUPSD instead. There is a newer version of this being maintained by an active community developer. You should switch over to their version, if you need it. This may only be required if you plan on going to 6.12.x series once it releases.
Those were my two largest updates that I needed when upgrading.
Oh, I did have to upgrade my version of PUTTY on all my other windows systems so I could SSH into unraid. Something about newer cipher algorithms or certs being used and removing compromised older ones. Without this client software update it couldn't negotiate a connection.
At the same time I also switched my dockers from macvlan to ipvlan. That was painless for me, but I did need to adjust the docker config to remove the old params and use the newer params in order for them to start.
For my VMs, I pulled down newest vm tools and made sure to update them inside the VM after the unraid update.
I may have been lucky, but going from 6.9 to 6.11 was uneventful for me.