Although there has been an effort to deprecate the fields of the Tcl_Interp structure as not being suitable for public use, because there is a backward compatibility macro that people can set, there is a danger that lazy programmers will just set it and then ignore the fact that they are using something that will go away. Then they'll get caught out and gripe (unfairly) that we didn't warn them. Programmers are like that.
If we set it so that (when we're using gcc) we use 'deprecated' attributes on the result, freeProc and errorLine fields of the Tcl_Interp (i.e. so that if people don't define USE_INTERP_RESULT and USE_INTERP_ERRORLINE they guarantee that they won't get warnings) then such code as has a problem will become noisy and inclined to be fixed. People needing higher levels of assurance who are using -Werror probably ought to move to the new APIs sooner so as to get a longer guaranteed support period.
More info about the right sort of attributes is on:
Search for 'deprecated' for what I'm talking about using.