It seems like only yesterday that Magnar Sveen introduced his Dash library for working with lists. It provided macros and functions for dealing with lists that weren’t covered by the standard Elisp library. Dash gained immediate traction and became a mainstay for Elisp developers. It’s one of those things that you use without even realizing it. So many packages install it that it’s present in almost every user’s Emacs so it’s easy to use its functions without realizing they’re from Dash.
Dash was so successful that its functions or their clones have made their way into the standard Elisp libraries. Now, Bozhidar Batsov writes that Dash has been essentially replaced by seq.el and subr-x.el and is no longer necessary.
No longer necessary except for the huge number of applications that use it. Batsov recently took over maintaining Flycheck and decided to eliminate the Dash dependencies. That turned out to be easy—almost trivial—to do. It was pretty much a one-for-one replacement of the function name.
It’s a tribute to Sveen’s vision and work that his library was so successful that the Emacs maintainers felt obligated to import its functionality into Emacs core. As a practical matter, Dash will live on if only because many of the applications that use it won’t have a Batsov to refactor them. It’s a nice library and I wouldn’t feel bad about using one of its functions today.