Relevant issues:

Issue 50133

Relevant PRs:

Deprecated in #50546

Removed in #98543

FlatButton, RaisedButton, & OutlineButton

Supported by Flutter Fix: no

The FlatButton, RaisedButton, and OutlineButton widgets were first
deprecated in v1.20, and then extended in v1.26.

They are replaced by new buttons, TextButton, ElevatedButton, and
OutlinedButton. These new widgets also use new associated themes, rather than
the generic ButtonTheme.

FlatButton

ButtonTheme

TextButton

TextButtonTheme

RaisedButton

ButtonTheme

ElevatedButton

ElevatedButtonTheme

OutlineButton