At least once, everyone should see how their body operates with as few confounding variables as possible. Your baseline performance, feeling, mood, energy - is very valuable to know.
Elimination Diet - Remove as many variables from your total intake as possible. Ideally choose a single bioavailable food prepared very cleanly for 30 days.
This is important because lots of people don't know what they have normalized as "getting older", or "I've always had that", or "I'm just inflamed". Skin conditions, hair issues, attention, clarity, are often reported to resolve on these type of protocols.
The most famous elimination diet demonstration was with Celiac disease during WWII, wheat/bread shortages created a accidental elimination diet in Denmark and established the link between wheat protein and Celiac.
Metabolism touches every part of the body, including the brain. Metabolism is driven by diet. Food literarily affects every part of our lives.
I think people should be aware this is a very useful tool, and if there is some persistent or difficult to nail down issue - why not try it?
elimination options
Omnivore options - eggs, red meat are good options. Ground meat has higher histamine levels, so it would confound the results.
Pescatarian - fish are ok, but they are not biocomplete, but that shouldn't be a problem for 30 days. The "sardine fast" is a type of elimination diet protocol.
Plant Based - low fodmap diets probably eliminate the most variables, but I'm not very well read on the options
Fasting - DO NOT DO LONG FASTS WITHOUT MEDICAL SUPERVISION. Probably the the most extreme option, total elimination, not exactly your base line, but if something was bothering you in your food you would at least notice it. REFEEDING SYNDROME is a real thing, and needs to be planned for when ending the fast.
Regardless, be aware of confounders - cooking oils and fats can change also be triggers, so be deliberate in your choice. Spices, seasonings, rubs, "electrolyte mixes", marinades introduce more variables.
My Biases
I run the ketogenic and zero carb carnivore (Which is just a elimination diet I decided to live with) communities, I'm all in on that metabolic lifestyle. However, elimination protocols don't have to aligned with my biases to be effective. Even doing something as simple as 30 days without processed foods can be helpful to know for someone.
Judy Cho wrote a great eliminate diet protocol book The Carnivore Cure Which is just eating red meat for 30 days and mapping out symptoms, mood, feelings. Plus guidance on starting, and reintroducing foods to nail down triggers. But, there are many different protocols out there, you can find one that fits your requirements.
I don't think you understood the point of an elimination diet. It's not about what you eat but about what you stop eating.
If you stick to meat for 30 days and start feeling better, it's not necessarily because meat is good for you - it's because you stopped eating something else that your body was responding badly to. After 30 days you can start adding other foods back in one by one. If you suddenly notice a decline in how you feel, the last thing you added is the likely culprit.
It's much easier to remove everything and reintroduce them one by one than to randomly cut out certain things to see if that makes any difference.
Everything in my comment was about their carnivore diet, not about an elimination diet – the lone exception being the last sentence advising not to start an elimination diet based on their advice without reading actual scientific/medical literature, as the OP is the opposite of a credible source for nutrition. (What I should've said too is to consult a doctor or a registered dietitian; people are generally not great at personally poring over primary or even secondary medical literature to decide what to do with their health, although the latter is far preferable if you do so.)
I.e., I was not making any claims to the healthfulness of an elimination diet; I have not done enough research to have any kind of credible opinion on it, which is why I chose not to express one except to advise that people actually research first (or, as appended in this comment, ask an actual expert).
Edit: I will express that the specific diet the OP proposes of "one bioavailable food for 30 days" is absurd on its face and obviously dangerous. Low FODMAP can potentially be quite good for e.g. lower GI problems, but I know little of any others.