In any but the most vestigial and nostalgic way...?

Vestigial in this context means all but a very small remnant of something that was once much larger or more noticeable.

vestigial adjective: forming a very small remnant of something that was once much larger or more noticeable. "he felt a vestigial flicker of anger from last night"; synonyms: remaining, surviving, residual, leftover, lingering. see, Google

Nostalgic is used to mean characterized by or exhibiting feelings of nostalgia. i.e., expressing a romantic or sentimental sensibility.

nostalgia noun: a sentimental longing or wistful affection for the past, typically for a period or place with happy personal associations. "I was overcome with acute nostalgia for my days in college"; synonyms: reminiscence, remembrance, recollection. see, Google

So, Mr. Hitchens is expressing the opinion that we shall not be hearing much of the gods Pan and Osiris (or any of the thousands of gods who once held people in utter thrall) because reason has vanquished them and all that is left of these gods is a very small remnant (i.e., in literature and myth), therefore only someone suffering from nostalgia, who has a romantic, sentimental and backward-looking understanding of the irrational concept of supernatural deities, is likely to discuss "the gods" at all.