The 道德经 was not written in the last century, and the ancient pronunciation is only approximately known. Yes it's written in Modern Standard Chinese/pinyin as "Dào Dé Jīng" but the text has existence in the western world older than the Modern Standard Chinese language, certainly longer than modern Chinese orthography.

Looking at Zhengzhang reconstruction of the title, for instance, we get the pronuciation /l'uːʔ tɯːɡ keːŋ/ (I don't know old Chinese phonology at all, I'm just working from wiktionary - please forgive any errors/take with a grain of salt). I don't see any particular reason for English-speakers to use the Modern Standard Chinese pinyin orthography/pronunciation to write terms that come from a considerably older way of speaking. (I say this as someone learning Classical + Middle Chinese using Middle-Chinese pronunciation).

Okay one possible reason is that it might be seen as good if the main inheritors of the tradition (the modern Chinese state+people) get given 'ownership' of it, and that outsiders speak using their preferred terminology/pronunciation. But I'm not personally on board with that, any more than I'd insist that people pronounce Shakespeare in American English.

[ I apologise for any snark that might be residual in this reply (and acknowledge that the remark is slightly tangential to the topic of this page) - I've tried to keep it constructive. ]

My Daoist teacher doesn’t really mind either way, although his english usage is the “Dao” form. I am assuming that is the more modern/current form.