I created a new database based on an existing but replaced all datetime columns by datetime2(2) and do not want to show this as difference neither want it be rolled back, when I create deployment scripts.

Same could happen, if I change the size of a varchar, decimal or integer datatypes (bigint <> int <> smallint ...) and even, if a number is in one database is stored in a varchar field, I could want to ignore this and deploy only everything else

See also
https://redgate.uservoice.com/forums/141379-sql-compare/suggestions/7109654-an-ignore-option-for-differences-between-varchar
which suggests to ignore varchar vs. nvarchar

BTW: I solved my problem by syncing the source database with an empty script folder, used Notepad++ to replace all [datetime] by [datetime2] and finally compared the script folder with the target database.