Yesterday I noticed that Google Scholar forgot one of my articles from 2018, on arXiv. See: https://scholar.google.com/scholar?q=arXiv%3A1811.04960 Google Scholar is not the same as Google Search, which can still find it https://www.google.com/search?q=arXiv%3A1811.04960 For how long, I have no idea. The article was at the same link all the time and arXiv is very reputable.