What's the problem with UX now compared to before?
Then in 2012 or 2013, they 'modernised' it into the current one: glued pictures with almost no separation and 100% screen occupation making an irregular patchwork; no title/description if you don't hover, I guess to accommodate a growing number of people who dumped their memory card without captioning their photos, and often without sorting them.
When Flickr repositioned post Yahoo acquisition, it felt like it was aiming for more of a Google Photos-type use-case that was very passive and "it just works", which felt very different from how it had been popular before. I think history bears out that this was a poor decision, we don't really have major players now that glue those styles of usage together like Flickr did. I think it just required a higher level of effort in both the software and the users, and once Flickr pushed away the audience that enjoyed that level of effort to try to acquire a more set-and-forget audience it lost the first and failed to appeal much to the second.
I think the old core Flickr userbase often went to 500px, which I don't like as much but is closer to Flickr in the good old days.
Hmm... Checking the 500px's homepage and profiles it has the 'modernised' points that wott mentions.
That and the attempt to introduce social media aspects: you got notifications that said 'here are some people you might know'. Which I never did. If they said 'here are some photographers whose work you might like' it would have been closer to the original intent.
Lots of annoyances like these drove me away from the site, and I never went back (or renewed my pro membership). Over time, they gradually undid the worst changes, but it was too late by then.
One thing that I did take away, though, was my Flickr username [0], which got repurposed for HN :-)