I don't think that happens all the time. I had two seniors that passed away from strokes. Up until the strokes drastically limited their ability to move they never had issues with getting to the litter box. They never had accidents before this. I ultimately had to put my babies to sleep because their quality of life was completely gone due to the strokes.
One of my current cats who is not a senior yet stopped using the litter box to pee once but it was because he had a UTI. Once that was treated he went back to using the litterbox completely.
The senior cat my parents had never peed outside the litter box at all.
I think that when any cat does pee outside the litter box the first thing that needs to be done is a vet exam. In an older cat that has never had litter box issues this may be a treatable medical issue. The cat my have a UTI. The cat may be getting arthritis and may need an extra litter box in a multi-level household.
My first kitty did. It was rather amusing to watch her jump into the litterbox and have the pee shoot out onto the floor I ended up cutting a box or plastic and filling it with litter as a 'catcher' tray. She was well into her senior years---17 and diagnosed with CKD. I didn't care. I loved her.
So far all of my old cats had kidney disease, so were unusual circumstances, but each one tried very hard to get to the box. ONE would have two feet in and pee, so I couldn't do anything but praise him
. The other never missed the box except with she had a UTI, similar to darkhorse's experience, so I would say the answer is "no, not all old cats stop using the litterbox".