Thanks for your reply. But as you say, it's not available for custom text boxes as it used to be. In accordance with best practices for slide presentations, I avoid canned templates whenever possible--after the title slide, very few of my slides use the pre-programmed title and subtitle fields.
Apple had a perfectly serviceable control, easily accessible and applicable to all text fields. Then they decided to bury it 4 clicks deep and make it available only to the fields THEY think you should need. Theirs.
What's more, in my version of Keynote (6.5), if I followed your directions correctly, it's not even there for the canned fields. I attach a screenshot. Am I missing something?