People eat keto before fasting because it makes it easier, not because it actually affects the effectiveness of the fast. I wouldn’t worry about it.
The one exception being ending a very long fast. You want to be more careful with long fasts (I think multi week fasts and above but am not exactly sure on length) you can see refeeding issues with an electrolyte crash, which can be unsafe.
Oh wow this brings back painful memories. I did Keto and intermittent fasting for a few months religiously. Everything felt great. Once I switched back to carbs and eating Arby’s I immediately started having excruciating pain at night where sleeping was not an option. The only relief was laying in a hot bath tub on my side. Pain immediately returned exiting the tub. I refused to go to doctor knowing they would remove something. I have no family, so I was just like if I’m going to die, I’m going to die. After I fasted for a few days, I finally was able to eat heavy carbs again with no problem. It was something I’ll never forget.