While helping a new developer here at Sony, I noticed that the "13.2 SSA Operands" section in the GCC's internals manual still refers to statements that were tree types.
For an example:
If you wish to iterate over some or all operands, use the FOR_EACH_SSA_{USE,DEF,TREE}_OPERAND iterator. For example, to print all the operands for a statement:

     void
     print_ops (tree stmt)
     {
       ssa_op_iter;
       tree var;
     
       FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS)
         print_generic_expr (stderr, var, TDF_SLIM);
     }

That refers to tree statement rather than gimple_stmt.
There are more examples in this section which needs to be updated too.