Answer
Yes and no.
Yes, List item IDs are preserved when using ShareGate Desktop's Insane mode for Microsoft 365 as this uses Microsoft's new migration API which allows these values to be set in a new list.
No, if you are running migrations without Insane mode enabled, or migrating using Insane Mode to an On-Prem destination. This is because we will use the regular SharePoint API, which assigns these values automatically.
You can see List item IDs for more information.