[layer8]

“Tsu” is the Hepburn romanization, “tu” is Kunrei-shiki (and the older Nihon-shiki). The latter is nominally the official standard Japanese romanization, though there is currently a proposal to change this to Hepburn, which in practice is much more commonly used. This is strictly about transliteration, not about pronunciation. The pronunciation has always been “tsu”.

You are right about the transliteration of English words into kana.

[thaumasiotes]

> The pronunciation has always been “tsu”.

This seems unlikely. The pronunciation was tsu before WWII. But the organization of the syllabary strongly suggests that the pronunciation was once tu.

I don't know to what degree the syllable 'tu' is viewed as impossible in Japanese as opposed to merely nonexistent. (Compare Mandarin, where (as in Japanese) there is no syllable /si/, but it's not especially difficult for Mandarin speakers to pronounce /si/.) I'd be interested if you knew.

[layer8]

From what I’ve read, it’s been “tsu” at least since the Heian period (so for roughly a millennium), and there is no clear evidence that it has ever been “tu”.