he said "let's go, deku!" on the page where almight gave him the tool he can use to continue being a hero! it's not surprising he'd call him deku again cause that's basically his hero name too, right? ^^
That was special though, Bakugou's use of 'Let's go, Deku' was to let us know that Bakugou wanted Izuku to join him in hero work and used his hero name, it was a significant usage of it! He used 'Izuku' only from the chapter he apologized in 322 until 430 so idk why it would change for one single seemingly insignificant moment. I could be wrong though! Or maybe it might be a mistranslation because the translators are so used to using Deku.
It's canon that Izuku wrote about Bakugou in his hero journals! Well, at least his high school journals! Also, the time set must be pre-Bakugou apologizing to Izuku because he is using Deku here.