>>>> (defun texinfo-prettify-symbols-compose-p (start end _match)
>>>>    ;; We know the matches all start with a backslash and end with
>>>>    ;; a word-element.
>>>>    (not (or (memq (char-before start) '(?\\))
>>>>             (memq (char-syntax (or (char-after end) ?\s)) '(?w))
>>>>             (nth 8 (syntax-ppss)))))
>>>> (add-hook 'texinfo-mode-hook
>>>>            (lambda ()
>>>>              (push '("\\alpha" . ?α) prettify-symbols-alist)
>>>>              (push '("\\beta" . ?β) prettify-symbols-alist)
>>>>              (setq prettify-symbols-compose-predicate 'texinfo-prettify-symbols-compose-p)
>>>>              (prettify-symbols-mode)))
>>>
>>> That is not completely correct because for "\alpha-\beta", only \beta 
>>> is prettified.
>>
>> No, \alpha-\beta is prettified correctly.  Tested with Emacs 25, 26, 27 
>> and 28.
>
> Have tried it in a texinfo document using
>
> @tex
> $\alpha -\beta \gamma \alpha - \beta \gamma \alpha, \alpha. \alpha- \alpha() \alpha[]$
> @end tex
>

This works correctly with vanilla Emacs, with the "defun" and "add-hook" 
above.