Oh, yeah, not arguing that Opus is the superior format. It 100% is. Not questioning that.
Indeed, the first place that gets hit by lower bitrates with MP3 is high frequencies. MP3 does have a pretty harsh cutoff at very high frequencies... that the vast majority of equipment can't reproduce and most ears can't hear. It's relatively debated, some claim to be able to "feel" the overtones or something like that. I'm extremely sceptical, if I'm being honest. Last time I did the test - must have been a decade ago - I couldn't distinguish lossless and high bitrate MP3 any more accurately than a coin toss.