My 84 year old husband who has dementia is suffering from Covid. His dementia seems so much worse. Is this due to Covid?

Neurological effects are common with Covid, it's been discovered, even with only minimal studies being done at this point. It's impossible to say whether your husband will snap back from this level of cognitive decline; only time will tell. His doctors won't know the answer to that question either, unfortunately, since no medical professionals have any answers to the myriad Covid questions everyone is facing. All you can do is deal with DH as he is now, taking things one day at a time and see how things progress as he heals from the virus.

I'm so sorry you are both going through such trials & tribulations. Sending you a hug and a prayer that time resolves these issues on all fronts and that DH gets back to a better baseline with his cognition.

My LO HAS HAD 2 separate COVID infections, and by observation, I believe she did sustain some cognitive/neurological damage in the process.

Her condition deteriorated quite significantly during the 6 or so months when we weren’t allowed to see her in person, and sadly, I don’t expect any improvement.

If all other factors in his life haven't changed, then it's safe to assume Covid's the culprit. Whether the issues will remain is anyone's guess, though, but I'd assume that he'll probably rebound to some degree but not to where he was pre-Covid.

My experience with my mom's dementia was that whenever she had any kind of health crisis her dementia increased. She didn't really decline at a steady pace but rather at a "down a step, then plateau" pace. The health issues caused the step-down, and she'd cruise along until the next one. She, too, had Covid (maybe Delta variant), and she declined, but it's hard to know if that was caused more by Covid or the two weeks she spent in the hospital prior to Covid. Covid was a breeze for her, but the hospitalization was far worse.

This is so sad, but we are probably going to learn about more cases like this as people insist that the Covid epidemic is over and that we shouldn’t vax, boost, mask or distance. I’m sure that your husband’s worsening brain health is due to having Covid. I know others who have the same issues. Research is ongoing and we don’t know nearly enough, but the disease does affect people with dementia and causes a downslide in cognitive ability. Not always, but in enough people that the US VA and University of Cambridge are studying it. We all need to be careful not to make others sick and we shouldn’t stop taking precautions. OP, I hope your husband recovers. I’m not feeling optimistic about it, though, based on others who are experiencing similar issues.

Brain fog is common and there are many with MRIs that now show loss of brain matter and yes indeed, his condition is likely now much worse DUE TO covid. Also think in terms of the breathing and the oxygenation of the brain. Encourage breathing exercises. Long breath in through the nose and let all the air out through the mouth.
He may snap back but many WITHOUT impairment are taking a long time to come back due to long covid. I just saw a program where people are actually retraining the brain to function by mindfulness to the task and with of all things computer games that encourage training the brain to stay in the moment and "concentrate". It tends to get frantic and scattered after covid.
I sure wish him good luck. Hopefully once the covid is over there will be no "long covid" but when there is the beginning of dementia anything like this can worsen it; you cannot know if permanently or not for some time. Anesthesia can often have dire repercussions.
Hope you will update us as you go and again, good luck to you both.