Loss of taste and smell.

Loss of Smell and loss of taste are one of the early signs of a Dementia. Thats why lose suffering from Dementia like sweets. They seem to be able to taste sugar and its instant gratification.

Loss of sense of taste and smell is common in covid. You would not be able to know, as you say, when this occurred for her, or whether or not covid is involved. Loss of taste and smell also is common for some Dementias, including Lewy's.
The internet is full of cases and information of loss of senses of taste and smell in the elderly, so you can find a lot online. For instance:

"Loss of Taste and Smell - Cleveland Clinic
https://my.clevelandclinic.org/health/symptoms/16708
Aging often contributes to a diminished ability to smell and taste. Other causes include: 1. Brain injuries. 2. Cancer treatments like radiation therapy. 3. Dry mouth from Sjogren’s syndromeor other conditions. 4. Exposure to chemicals. 5. Gum disease(gingivitis) and other dental problems. 6. Medications, … See more".

It's been proven "that people with Alzheimer's can lose their sense of smell and taste many months or even years before other symptoms, such as memory loss become apparent."
The loss of smell and taste is often a precursor to dementia.

This can also be a side effect of some medications.

For what it’s worth, post-covid I can taste but I can’t smell.

Old,

Sounds awful. No wonder that some elderly people don’t have any desire to eat. They aren’t able to enjoy taste or find pleasure from eating anymore. Very sad 😞.

As the dementia progresses it affects those areas of the brain that register smell, taste, fullness, and hunger.
My mom just had covid and her appetite, which was waning, is now pretty much gone, so I think the covid made it worse.
I can sympathize--it was very odd after I had covid to make something for dinner and ...nothing. No taste at all. It really does focus you on the textures.

Yes, it’s a symptom of dementia.

My dad passed away in 2005 so no covid, nor did he have dementia. However he did have to take very strong meds for a period of time. He lost a great deal of weight. One day I asked him how something tasted to him. He laughed and said “Taste, I haven’t been able to taste anything in awhile”. He had never mentioned this to us before. But he did enjoy certain textures. Ice cream was one which helped him gain his weight back.

My FIL, also passed precovid, couldn’t smell and could not believe others could.🙄