Doctor prescribed hydroxyzine. Do you know where I can find information about, "Drugs one should not take with a dementia diagnosis?"

You could ask your Pharmacist. What I found says it won't cause Dementia. The other thing I found...

"Older adults should not usually take hydroxyzine because it is not as safe as other medications that can be used to treat the same condition. you should know that this medication may make you drowsy.Feb 15, 2017"

Nearly any med will have warnings about safety for elderly and/or those with dementia. You need to consider the risk, but also the potential benefit. For example, Seroquel is a med that is often used for those with dementia to treat anxiety. It is prescribed off label and is very effective for some with anxiety, and some not at all.

While anticholinergic drugs are thought to perhaps cause dementia, prescribing an antihistamine like hydroxyzine will not likely "worsen" your mother's dementia symptoms! In fact, Dr Google says :

Can antihistamines make dementia worse?
While certain types of anticholinergic medications were linked to dementia, antihistamines were not.

Antihistamines may be of great value in helping mom with itchy skin. If not, THEN discontinue their use. Every medication has pros and cons, of course. If this med cures incessant itching, that's a huge benefit. Geriatric pruritis, which is itchy skin, is quite common for elders especially those suffering from dementia. My mother has 2 solid years of insisting she had a horrible rash on her breasts and thighs that was very itchy. Nobody could see these "rashes" however, and her poor doctor prescribed every med known to man to help her, but nothing did. I wound up buying her a can of Dermoplast at Walgreens one day and sprayed it all over her "rashes" and voila, the itching stopped. Dermoplast has lidocaine in it. So if this RX doesn't work for your mom, I recommend an over the counter product with lidocaine in it to numb the itchy area.

Years ago, mom had bad vertigo and I begged the doctor to give her a Scopolamine patch when she was hospitalized with pneumonia and carrying on about vertigo. I'd been dealing with ENT docs, ER visits, and you name it for another two years for vertigo which also had no good results, ergo, the request for the patch which I'd read was a cure. It was, too....the vertigo blessedly STOPPED on a dime the same day the patch was placed behind her ear. I all but dropped to my knees in gratitude to God. Until she started seeing MICE running around on the floor in her room when naturally, there were no mice. Off came the Scopolamine patch and back down the rabbit hole we fell with vertigo yet again. As it turned out, Scopolamine is known to worsen dementia symptoms for real. 🙄

I would be more concerned with getting the itch controlled than the antihistamine issue, if this were me. Pick your battles bc they're ALL battles with dementia at play. I will send up a prayer that your mom's itchiness is resolved with her RX without further ado.

Good luck and Godspeed as you travel this journey.

BEERS LIST of drugs not recommended for elderly.

Antihistamines Antihistamines, anticholinergic (B): Chlorpheniramine (ChlorTrimeton, etc), Cyproheptadine (Periactin), Dexchlorpheniramine (Polaramine), Diphenhydramine (Benadryl, etc),
**Hydroxyzine (Vistaril, Atarax)**,
Promethazine (Phenergan, etc) Anticholinergic AE, urine retention, confusion, sedation5 Cetirizine (Zyrtec), fexofenadine (Allegra), loratadine (Claritin), desloratadine (Clarinex), levocetirizine (Xyzal), low-dose diphenhydramine19,26 A

I think Teepa Snow may be the person you are thinking of.

Here is a link to an article about benzos, anticholinergic drugs and dementia:
https://www.health.harvard.edu/mind-and-mood/two-types-of-drugs-you-may-want-to-avoid-for-the-sake-of-your-brain