SharePoint Online (SharePoint for Office 365) and SharePoint 2019 support the special characters # and %.
SharePoint Online and Office 365 require activation in order to enable special characters.
To activate this feature:
Install the SharePoint Online Management Shell
Connect to SharePoint Online using Powershell
Launch the following command:
Set-SPOTenant -SpecialCharactersStateInFileFolderNames Allowed
Note: This procedure takes several hours to activate in the environment.
To enabled this feature:
Open the Sharegate Desktop, go to Settings and click Migration.
The Allow Office 365 special characters option in Settings -> Migration must be toggled on.
Note: You must run a migration to test if the feature is properly activated. There is no other way to know at this time.
If you try to preserve these characters in an environment that does not allow it, you will receive an error when migrating.