Anyone can upload a CC-BY article in full to anywhere, and anyone can upload a CC-BY-NC-ND article to anywhere noncommercial. ArXiv only accepts uploads from authors, though.

The "history" section of the Wikipedia article cites Kay's excellent "Early History of Smalltalk" https://dl.acm.org/doi/pdf/10.1145/155360.155364 which of course does cite Ingalls's 01978 POPL paper, as well as 17 other papers published by the ACM, by my count, more than any other single publisher except Xerox. That section also highlights the ACM conference OOPSLA and cites Borning's "Thinglab", published at OOPSLA. So access to historical ACM papers is extremely important for understanding the history of object-orientation.