You should never, never, never drop columns or tables in the SYS schema. The SYS schema is a special schema, containing the Data Dictionary. One should never drop tables or columns for Data Dictionary tables as this can lead to Data Dictionary corruption and a potentially unusable database. And one should never create their own tables in the SYS schema. If you have your own tables in the SYS schema, move them to a new schema. Then you wont have any problems dropping columns.
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