Hey, it's 2021 and this issue hasn't been fixed on the main opera build… but I found a way that doesn't nees to tweak with the Experiments (I also don't think Widevine is available there anymore).
First I opened the Opera Components tab by typing chrome://components, but the update function did nothing. I went to the extensions store but couldn't find Widevine Content Decryption Module there, so basically I installed the "Install Chrome Extensions" and tried it again.
It took me a while to figure out that I'd have to relaunch Opera before I would enable the update check, but it worked. So basically all I needed to do was install that official extension by Opera, relaunch the program and update Widevine on the Components. Not bad for someone who just started using Opera for Windows today, huh?