I have the DT990 Pro as well, though mine was purchased in 2016 and hasn't broken with near daily use. However, Beyerdynamic sells replacement parts if you google it. This specific repair looks to be fixable with their 'slider repair kit'.

I actually managed to 3D Print new ones, with metallic blue filament - not as rugged as the originals, but they work and look awesome!

I tried to buy the official Beyerdynamic kit however its always out of stock, I ended up purchasing 3D printed ones from Ebay and painted them black, Im not sure aboyuyt other reviewers however mined are MUCH stronger than the OEM version

The replacement parts are on eBay (in the US) right now for $12. Mine broke like this after about 10 years and it was a simple repair. I've also replaced the earpads. The repairability of these headphones means they will last a very long time.

You can get a replacement from beyerdynamic. Its a 15 mins repair. Beyerdynamic replacement parts: https://europe.beyerdynamic.com/slider-repair-kit-black.html 3d pritner file: https://www.thingiverse.com/thing:831032