Does minifying and concatenating JS/CSS files, and using sprites for images still provide performance benefits when using HTTP/2?

They're still useful. HTTP/2 reduces the impact of some of these practices, but it doesn't eliminate their impact.

Minification remains as useful as ever. Although HTTP/2 introduces new compression for message headers, that has nothing to do with minification (which is about message bodies). The compression algorithms for message bodies are the same, so minification saves just as much bandwidth as it did before.

Concatenation and sprites will have less of an impact than before, but they will still have some impact. The biggest issue with downloading multiple files instead of a single file with HTTP/1 isn't actually an HTTP-side problem, per se: there is some bandwidth-based overhead in requesting each file individually, but it's dwarfed by the time-based overhead of tearing down the TCP/IP session when you're done with one file, then starting up a new one for the next, and repeating this for every file you want to download.

The biggest focus of HTTP/2 is eliminating that time-based overhead: HTTP/1.1 tried to do this with pipelining, but it didn't catch on in the browser (Presto is the only engine that got it completely right, and Presto is dead). HTTP/2 is another attempt, which improves on HTTP/1.1's methods while also making this kind of thing non-optional, and it stands to be more successful. It also eliminates some of the bandwidth-based overhead in making multiple requests, by compressing headers, but it cannot eliminate that overhead completely, and when downloading multiple files, those requests still have to be made (as part of a single TCP/IP session, so there is less overhead, but not zero). So while the impact of concatenating and spriting is proportionally smaller, there is still some impact, especially if you use many files.

Another thing to consider, when it comes to concatenation and spriting, is compression. Concatenated files of similar types tend to compress better than the individual files do, because the compression algorithm can exploit similarities between the concatenated pieces of data. A similar principle applies to sprites: putting similar images in different regions of the same file usually results in a smaller file, because the image's compression can exploit similarities in the different regions.

So far, all the answers tacitly assume that you'll want to download ALL the .CSS and .JS files for every page. A benefit from using http/2 and keeping .CSS and .JS files separate is that you can only bring down the ones you need, and not downloading something is always faster than efficiently downloading it.

Yes, it is still useful.

Along-side with gzip compression, you page will weight less.

Imagine you are using a very slow GPRS (56Kbps, 500ms ping) network.

You have 50 tiny images, 30 javascripts and 20 css files.

This means that, with 2 parallel connections, you must wait over 100 * 500ms just for the requests.

Now, each image is about 3-4kb. Which might take a few milliseconds (5-8?).

Now, the CSS files and Javascript range from 20Kb to 600Kb.

This will kill your website with a huge transfer time.

Reducing the time to transfers the files will increase the 'speed' at which the website will load.

So, YES, it is still useful!