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.
For more information, see List item IDs.
Note: When using Copy content, if you delete items on a destination list, the Microsoft migration API will not reuse those list item IDs. For example, if you delete the list item with list item ID 2 at the destination then no list item will have list item ID 2 following your copy content procedure. The workaround is to use Copy structure instead.