I don't think enough mentally healthy people know enough about dementia. It is generally known that dementia affects or can affect someone's memory, speed of speech, or difficulty recalling the right word at the right time, etc., and that it is progressively deteriorating.
When my wife was diagnosed with Alzheimer's, she said in no uncertain terms that she didn't like that word nor the word Dementia. She did not want me to tell anyone that she'd been diagnosed. However, as time went on, she didn't dislike being told she had "tangles" in her brain that affected communication; and as time went on she realised she had deteriorated in what she could do, and so she accepted my view, that is, to tell people so that they would better understand how to deal with her and answer her sometimes incoherent questions.
It was at that stage that I found out many of the family and our friends (who often said they knew someone with dementia) did not realise that this disease affected not only my wife's memory and recollections, and word finding, but also her concentration (so she couldn't read novels), her co-ordination (so she couldn't write or play the piano or guitar to her previous high standards), her confidence (because of these other "couldn'ts"). Mobility and steadiness and standing for any length of time became an issue.
Everybody "knew" Alzheimer's and Dementia was a mental health illness and issue, but none realised it is a physical illness that affects mental impairment.
My wife would not have wanted to wear a lanyard or bracelet that would have identified her as different to "normal" people as initially she didn't want to be differentiated. Later in the downward progression of the illness such a symbol might make sufferers a target for the unkind and dishonest, rather than as someone to help. A person who is vulnerable doesn't in my opinion want to advertise that.