My dad drinks in his care home. Could this be causing cognitive decline?

Your father is 91 years old. If he wants to drink and the AL he lives in allows drinking, then I say more power to him at his age. The fact that at his age he can still throw them back is amazing and remarkable.

He's in AL and being looked after. At 91, he's going to die of old age not alcoholism. So leave him alone and let the guy enjoy his drinks.

Yes, they could. Unfortunately, some people want to drink themselves to death. It's happened in my own family, and it's tragic, but that's how powerful addiction can be. You could back off, if you like. If it's too painful to watch, that's what you may want to do. There are no easy fixes.

As Geaton said he could have wet brain.

"Wernicke–Korsakoff syndrome (WKS) is caused by a thiamine (vitamin B1) deficiency that damages the brain. Thiamine is vital for brain cell function, and a lack of it can have severe and long-lasting effects. Alcohol abuse is the most common cause of WKS in the United States, as alcohol prevents the body from absorbing enough thiamine."

It can be cured if treatment is given early on. He may have to stop drinking.

You really don't tell us much here.
If Dad is allowed to drink in his facility, then he can drink in his facility.
Many ALFs allow alcohol and in fact serve wine at "Happy Hours".
If intoxication were a problem for/with your father I imagine that you would be contacted by the facility and suggestions would be made for addressing this issue.

If your question is "Can Alcoholism cause cognitive decline" then the answer is that it MAY but we are talking severe alcoholism over time. That would be alcoholic encephalopathy. You can feel free to investigate that with Google or another search engine.

You say in your answers below that your dad was "long ago" diagnosed as an alcoholic. Then just assume that he likely still is one.
My brother's ex and good friend drank right through his own years in their ALF; was his dementia due to THAT or another sort of dementia? That's between him, his doctor and his POA. In fact, when alcohol was removed he made some super easy, super cheap cocktails from Listerine. The reason you see those enormous bottles of it on the end aisles in stores isn't that we all have dreadful breath.

I would make sure the administrator of the facility is aware of the situation, both the alcoholism and the alcohol being provided to him. As their resident, they deserve to be informed of both. After that, it’s on them to handle

Is alcohol permitted in his facility?

If so, then you can't stop it. You can let the staff know of his previous diagnosis of alcoholism so they can decide if they need to change their treatment of him.

If alcohol isn't permitted, you can let the staff know, so they can deal according to their rules with the person who is bringing it to him. You should also let the staff know of his history of alcoholism in case he needs to be gradually detoxed rather than cut off all at once.

You cannot cut him off cold turkey. He will need detox

Since you don't give us much to go on I will just say that the fact that your dad is 91 and the fact that he drinks could both be reasons for his cognitive decline.
And if he's been drinking for many years and is an alcoholic there is a dementia called Wernicke-Korsakoff syndrome that is caused from drinking.
Anyone drinking alcohol in excess is at a much higher risk to develop dementia.

Alcohol abuse can cause cognitive decline.
I do hope he is not driving to get the alcohol.
Who is POA if anyone? It sounds like it is time to step in and remove the keys from him.
Has he caused a problem in the facility with the drinking?
Have they said anything?
Depending on how much he is drinking to remove all alcohol could do more harm than good.
And is his doctor aware of the drinking?
The alcohol combined with drugs either OTC or RX can cause problems so his doctor should be informed.

Yes. Alcohol consumption can definitely cause cognitive decline and dementia if consumed to excess. If dad lives in AL, he's free to drink alcohol if he'd like. He cannot drink in Memory Care Assisted Living or in Skilled Nursing.

I bought alcohol for my parents when they lived in AL and mom had early dementia. They did not drink to excess, however, and it was nice to see mom relaxed or happy once in awhile. There are differing views about this, but once a person hits 90, chances are good you are not changing them or their habits.

Sure could, especially if he has been a long-time alcoholic he could get Wernicke-Korsakoff dementia (aka wet brain). There have been several posts recently about alcoholic parents and what to do about it. The answer is: nothing. You can't have their recovery for them. And, depending on much they drink, may need a medically supervised detox so that it doesn't kill them. Who is buying the booze for him? Or, is he still driving? The person buying the booze needs to know they may be liable if your Dad gets drunk, falls and injures himself. No one should be buying booze for a senior with cognitive impairment since their ability to regulate how much they consume (or remember they consumed) can be faulty.