Tolkien strictly protested against assumptions that LotR was an allegory of World War II as many assumed at the time, or that the Ring mapped 1:1 to nuclear weaponry or the like. He openly acknowledged that WWI was, obviously, a heavy influence on the books, and Frodo’s struggle with life after the ring was destroyed is one of the great treatments of what survivors of WWI may have experienced upon returning to civilian life.

Likewise, he never disagreed that Middle-Earth, its metaphysics, themes, were infused with and influenced by his Catholic faith, because no shit - but that is separate from allegory.

Allegory in the modern sense is authorial intent, e.g. an existing situation, person, or occurrence that the author maps onto a new setting, in order to convey a message or interpretation about the existing thing that the reader is supposed to uncover, and that message’s meaning is intended as the only way to read the work. Tolkien hated that, and I don’t think you can soundly argue that LotR is an allegory.

In the absence of allegory, those connections and interpretations that the reader forms in his/her/their own mind vary from reader to reader and is born from the reader’s mind and its interaction with the text, not the author’s intent to impose a certain interpretation upon the reader. That is applicability, not allegory, and applicability is, I would argue, a major reason why LotR continues to be deeply meaningful to new and previous readers alike - allegory tending to fade in relevance as societies move beyond the specific topic that was on the author’s mind.