Is ECO, Encyclopedia of Chess Openings still considered the best printed reference work for all the chess openings, as it was back in the '80s?
There does not seem to be a consensus on this, but here is a relevant discussion:
For printed opening material (as you requested) I prefer ECO with Chess Informant updates for new opening ideas and analysis: http://www.chessinformant.org/about-chess-informant/
I can't speak on the usefulness of ECO for serious study. However, I can tell you that https://www.openingtree.com/ (which is not a printed reference, but is relevant to the title) is much easier than a book to search: just play the moves, and the most common lines with white/draw/black percentages are displayed. You can search by Lichess players, chess.com players, notable chess players, and notable chess events, as well as see the most common lines played and game results. The site touts a testimonial that "It's so powerful" from IM Eric Rosen. Lichess and chess.com offer similar opening explorers, with Lichess and chess.com having Stockfish integration in the browser.
IMO, this is an area that needs work. I developed a small computer program to check for duplicates in openings and found that most opening categorizations contain duplicate positions. For example, Schiller's opening list has duplicate openings. The same positions arise through different move orders but are named differently. ECO is far too general.
2I don't think the question was about opening names. Feb 3, 2017 at 21:20
4In some situations it's good to have those duplicate positions. If I want to study the Queen's Gambit, I don't know to go look 250 pages away every time I enter some line that could be reached from a Panov Attack at mvoe 15.– DavidJun 17, 2021 at 9:35
Maybe, but it doesn't mean much. Any version of ECO will always be an inferior, cumbersome substitute for an opening explorer.