That's a really interesting question, @Eden1919 . I'll be curious to see the mods and others thoughts.
Im also curious, if you are willing to share, your thoughts?
I personally dont have a set thought either way, except to wonder what the purpose of "self-diagnosis" and the severity of the mental illness is. So my thoughts would differ on the scenario.
If a person is negatively affected beyond their ability to self-manage (even after researching techniques that would be advised to a diagnosed person), then I feel that they would benefit from professional help (and that would by nature result in a confirmed diagnosis). If a person is able to self-manage effectively by using the techniques for the diagnosis they suspect they have, then that may be a more affordable and positive option for them.
I do not that self-dianosis could be wrong or could miss other conditions (and ive heard even professionals miss diagnosis too sometimes). E.g. a person may think they just have MDD but actually have Bipolar. Or they may have psychosis symptoms but only recognise anxiety because they believe/don't realise the psychosis-thoughts? (I dont know much about psychosis, so please correct and forgive me if that is not a realistic perception).
In terms of neurodivergence like ADHD and Autism (which I dont think is a mental disorder, but is in the DSM-5 and can commonly cause more distress to people which can result in depression, anxiety, etc), i think self-diagnosis is quite common among people i know - especially because of the costs involved with formal diagnosis. My personal stance is to always encourage people to identify techniques that help them personally - regardless of whether or not they would be diagnosable. But a formal diagnosis would help if they need financial assistance or access to supports only for diagnosed conditions.
Diagnosis terms are helpful for sharing a cluster of symptoms concisely. It can also be helpful for accessing funding and services (e.g. NDIS, DSP...).
But a diagnosis itself does not give any treatment in itself - which is why I take a practical perspective of using techniques regardless of formal diagnosis.