Is it still relevant? I say yes, but barely, and with caveats.
Also, beware of discussing the terms "50mm" and "Canon" together. There are strong opinions on this.
My breakdown is: Canon makes good optics mostly, not Zeiss, but good. The 50 1.8 in all its iterations is trash. The 50 1.4 is also trash. Complete trash. The motor is especially trashy, and it has a cheap plastic feel. Optically? Who cares? The front element moves in and out. It is junk. I have owned all of the 50's and my favorite is the ancient 1987 50 f2.5 macro.
Here is my 50 L story. Mainly with an L lens, I will NOT buy it unless it has 9 curved blades. (Case in point, I dropped my old 100 macro like a hot potato when the new 100 L IS came out because it had 9 curved aperture blades, and I find the difference between the 2 night and day. Bokeh? My word, the new 100 and its curved aperture all the way to f8 is AMAZING! And the glass is designed in a way that the front element does not move, and it still makes that gaussian 3D pop!) When I buy an L, I expect this quality. The 50L mostly lacks this quality. It has 8 blades, with very tiny curvature, and no curvature past f4. It has the Gaussian 3D pop, yes, but the glass is HUGE and the element moves in and out on a wonky weak-feeling motor mount. True Story: I bought my 50L because I had to have the gauzy dreamy f1.2 effect for portraiture. The first week I bought it, it fell a meter onto a heavily carpeted floor. Minor fall. Well, that was enough to break the motor, so I had to send it to Canon and pay $800 to fix it, and I get it back, and it makes a squeak noise when it focuses now. Sometimes manual focus fails, as in the glass doesn't move. It is the only L lens I regret buying, but there are no options when it comes to 50mm and 85mm on Canon, or at least at that time there wasn't. Now we have the 85 f1.4 IS, which is the best 85 they have, comparable to the 100 L IS.
The 50L was a triumph before we demanded good bokeh and attractive starpoints, but we're talking 1987 to 2005 era, and this lens came out in 2006. So it was in design 2 years before that, so this lens is now 14+ years old. In camera bodies, that is ancient. This lens was designed for a 14megapixel CCD full frame sensor (5d Mark 1, 2005)! On my 5DSr, it does not impress me at all, and compared to my ZEISS lens output on a Canon body? Even my cheaper Zeiss shame the 50L. Shame it! It is true that the Canon 50L is a specialty lens with its dreamy effect wide open, and that is a design choice that sets it apart from what the 50 f1.4 does, but for me to accept that to achieve that level of functionality the engineers had to accept some of the nasty cons the 50L has, then I am not sure about that. I have used other wide-aperture lenses that didn't have as many problems (aberrations, fringing, lack of IF, etc, etc).
A lot of good points. I dropped my 50mm 1.4 from about 2 feet up onto a soft couch and the focus track got bent and needed to be repaired. Lucy me one of the two lenses I have manged to drop since 1969
Canon seems to me to be making lenses for people who study resolution charts online sometimes. I got a 85mm Tamron f/1.8 when I got tired of waiting for Canon to get out their 85 IS. And I am so happy with it that I bought the Tamron 45mm. They both have what I would call creamy (as opposed to nervous) bokeh. And they are plenty sharp, easily in league with my 100L. A little slow focussing and f/1.8 only but they accurate and good stabilization. For people I would say consider taking a step back and using one of the excellent 85s like the Sigma or the Tamron.