It's not really the function that was deprecated, but the module it was in.
The same function can now be imported from a different module.
To figure out the new import, one can have a look at
and click on the commit id in the "Branch" field to see the replacements
that were made when the module was deprecated.
This is the new import:
from sage.structure.element import is_Matrix
How to figure all that out from scratch:
try the initial import:
sage: from sage.matrix.matrix import is_Matrix
then look at the source code of isMatrix
:
sage: is_matrix??
this tells you everything happens in the file .../sage/matrix/matrix.pyx
in a text editor, open the file <path_to_sage_root_folder>/src/sage/matrix/matrix.pyx
in this file there is a note about deprecation and a reference to Sage Trac ticket 24096
then see above.
Simpler way to test if something is a matrix:
sage: A = matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
sage: isinstance(A, sage.structure.element.Matrix)
True