Is it considered necessary to include a constant term in the definition of a panel data model which. among other regressors, includes a variable representing fixed effects?
The intercept should almost always be included, that is, if you do not include, you should know why. Specific reasons and discussion can be found in the similar question When is it ok to remove the intercept in a linear regression model?, which is almost a duplicate.
In comments is mentioned the case of a categorical predictor for $k$ groups, with all $k$ dummys included. But that is not really an exception, because in that case the intercept is implicitely included, as the column space of the design matrix have the constant vector.