Flat frequency response in plugs is nonsense, because due to factors like the Fletcher-Munson curves, there is no correct profile.
Foam plugs attenuating more of the high end means that you hear a lot of bass: and in one way that is subjectively pleasing, because due to Fletcher-Munson, the unattenuated loud sound sounds bass-heavy. Thus some of that aspect of the sound is preserved.
People who are cork sniffers about flat frequency responses should be going to Bach concerts, not rock concerts.
I heard Chick Corea's Electric Band (with Gambale on guitar at that time) via foam plugs. They sounded fine. I heard every note clearly from every instrument.
Most people don't give a shit about flat response. Just about every time I've ever rented a car, I had to correct the stereo from cranked treble and bass, reduced middle, back to all zero.
No two speaker systems sound alike in the first place.
So you do care about it after all?
For live music, there is no reference for correctness. The raw thing is so loud, it hurts. Nobody is engineering it for listening at moderate volume in an ideal listening space!
Now speaking of recorded music, even though there is a "correct" approach to reproducing it so that you hear more or less what the recording guys (I don't want to use the term "engineer" because often they are not) intended.
However, what those guys intended is often arbitrary. They took captures from some sources and tweaked them. The whole hi fi paradigm is predicated on the idea that the recording/mastering guys are high priests who get to exercise their taste, and the lowly consumer must obligingly reproduce what they put out.
This isn't so at a concert. If it sounds to me like some guitar dickhead's amp stack is putting out too much crap above 6 kHz (because he's standing out of its way, hearing it from the side or whatever), and my foam plugs make it smooth like it should, that makes me the "engineer" of my own experience of the event.
Don't be dismissive of audio engineers and mixers making records. I don't think you realize how much care and consideration goes into crafting an album. These choices are far from arbitrary for those who are working at the top of their craft.
Additionally, don't make the mistake of conflating people who are "hi-fi" enthusiasts with the people actually making the records. They are not always aligned in their goals.
You also seem to have the mistaken idea that mastering engineers are trying to impose their taste onto a record. You know which mastering engineers try to shoehorn their taste into a record? The ones who don't get hired a second time.
I am a mastering engineer. By the time that a record reaches our studio the creative decisions have already been made by the artist, the producer, and the mixer. Our goal at this stage is to function as midwife to the artist and present their music in the best possible light, which sometimes means doing as little as possible, so as not the get in the way of what's been delivered to us. Our role is to hear the record as objectively as possible and then give it a final level of polish/balance so that the artist's intentions shine through and translate on as many different playback situations as possible.
Finally, if a guitarist's amp is putting out too much 6kHz at a rock show, then it is the live sound engineer's responsibility to fix it, either through EQ or through communicating with the guitarist to adjust their amp settings.
Regardless of "flat response" talk, I have been very happy using the same pair of $30 Dubs ear plugs for almost 15 years. I have foam earplugs for when I need things to be as quiet as possible but I hate them so much for a few reasons:
1. With extended use, for whatever reason, cause my ear canals to feel raw and itchy.
2. They are a PITA to get in and have them stay in and sometimes to get them out
3. They are disposable which is a waste
Coincidentally, I literally just bought a pair of Loops ear plugs today as my Dubs are finally falling apart. I liked my Dubs for multiple reasons:
1. They are super comfortable - I forget I have them in
2. They are reusable
3. They look interesting in the ear which is a great conversation starter at concerts/clubs/festivals - "are you wearing earbuds?" "No they are musicians earplugs. They basically just lower the volume while preserving the sound" "Oh cool!" "Yea if you like music, don't forget to protect your ears!"
That's not really enough if you're going down from 110 or more.
Yes, Fletcher-Munson effects can change our perception of frequency response. But that doesn't mean that we should tolerate additional negative effects. That's like saying that we shouldn't stop the cook from using too much salt in our dish because the chicken is already dry. Clearly it's preferable to have chicken that is ONLY dry, rather than chicken that is both dry AND over-salted.
Don't be so dismissive about people seeking a good audio experience. If you're at a concert there's a lot of factors that are out of the audience's control, but good earplugs are low hanging fruit if such things are important to them.
And if you're the musician on stage cranking an amp, using cheap ear plugs is going to create a disconnect between what you hear and what your audience will hear. If you have to dial in more top-end to compensate for the dulling effect of the plugs for your amp to sound "right", then your "right" sounding amp setting will be perceived as harsh for the people in the audience who aren't wearing plugs.
Finally, if you're at a rock concert (with a sound engineer mixing the band) the engineer will be hearing the concert at the same volume as you. They will already be balancing the band's sound in a way that compensates for the effects of the Fletcher Munson effect that you're concerned about.