@Hollysaurus I'm sorry you lost your sweet Mapple.
I also had a very hard time when I dealt with my first chicken losses, even though they were not pets like yours. I found pet grieving support on the web and on social medias helped a bit, and talking to other chicken keepers who had learnt to deal with it in different ways. And poetry, like this well known grieving
poem from Mary Oliver.
Keeping chickens is emotionally hard because inevitably some will die much too soon, and it will make you feel like you haven't done enough, missed something, or have been guilty for whatever reason. But most of the times, it is just that we are actually powerless. Chickens can get very serious health issues, viral, bacterial or cancer. And sometimes it's just about their genetics and not about anything you have done. Keeping chickens means having to learn to cope with loss, in whatever way is right for you.
I'm also sorry you have gotten some pretty harsh answers. Sometimes people tend to believe their own choices, experiences, or things they have heard repeated over the internet are factual or scientific truths. For example, feeding only commercial feed and no more than 10% treat is not a scientific truth, it's a cultural north american belief. In my country, France, the most common advice given is 1/3 forage, 1/3 human scraps, 1/3 commercial feed. This is not more true, it's just a different cultural belief / way of doing things. If you have any worry about FLS read very carefully reliable source such as this one to see if it could relate to your chickens.
https://www.msdvetmanual.com/poultr...e/fatty-liver-hemorrhagic-syndrome-in-poultry
While an overload of worms could kill a chicken, it rarely does on it's own. And it seems to me that you would have seen the chicken grow gradually weaker ; not running for treats just a few hours before she passed.
My own experience of keeping a flock that has both tapeworms and roundworms, may not be typical, but it's that they can still be healthy with only minimal and individual deworming. Be sure as already mentioned if you use a dewormer, that it is one safe during moulting ; and be aware that if they do have a worm overload, deworming can sometimes be hard on their system by suddenly having all those dead worms to expel. If you can get a fecal float it would be best.
You've noticed your chickens have loss weight. Have you weighed them, or is this an impression? If you haven't, I would weigh them and check how the weight is evolving for the ones that worry you. Sometimes, chickens go off commercial food when they moult. I would also check that the amount of food they eat has not gone down significantly, and that there is nothing wrong with the food and the feeders.
And I would spend some time with them looking at their behaviour and symptoms. Loosing weight, diarrhea, combs changing colours, can all be attributed to various health issues, and even moulting. If you haven't already, do a full health check (if you're not sure what to include it's easy to find on the internet), especially crops, abdomens, and breathing. How are they acting ? Do they look uncomfortable, or actually sick ? How hard is their moult ?
You might find out more, but I agree that only testing can make you sure, or eliminate some of the most common virus. And even so. If Mapple's friends are only uncomfortable and unwell because they are moulting in very cold weather, you may just not find out why she passed.
Sending virtual hugs. Last, don't hesitate to write some kind of tribute post or show more pictures, if you feel like it. I've covered my rest room with pictures of my chickens who have passed.