Links Hierarchy is available on Cloud and Server/DC.
there is no export/import or migration option because the app (both Server/DC and Cloud) does not store any data on any third-party servers or on Jira. Therefore, you should have no problem migrating between platforms (Cloud to Server/DC, or vice versa).
The only thing you should keep in mind is:
If you had "Reverse Links" configured on the source platform, you must manually replicate them to the target platform.
If you want to migrate your app gadgets, you may need to replicate them manually. This is not particular to the app, there are users who reported problems to Atlassian when migrating any device from the server to the cloud and vice versa.
The Cloud does not support the development of JQL functions, so if you have filters on the Server/DC that use the app's JQL functions, these filters will not work in the Cloud.