i believe the mutating issue is when you try to update from anything other than the old; or new:

you want to avoid that.

it's not clear what you are trying to update; with no DDL, we can only assume all the columns referenced belong to the MODEL_CODE table the trigger seems to be defined on..

old: would be empty/blank for INSERT commands.

new: would be empty/blank for DELETE commands.

it looks to me like there is no reason whatsoever to try and use the other table USER_ID

CREATE OR REPLACE TRIGGER TRG_IUD

AFTER INSERT OR DELETE OR UPDATE

OF CODE,DESC

ON MODEL_CODE

REFERENCING OLD AS OLD

FOR EACH ROW

BEGIN

UPDATE UID SET DATE_SET = current_timestamp

WHERE UID_ID = :old.code;

END;