Holding onto the handles on a treadmill?

When you say "I saw that you shouldn't do that", what type of source did you see it on? Because people have different opinions that may or may not be based on data or a deep understanding of physiology, so not everything you read is entirely reliable. Also, sometimes doing things the 'wrong' way will cause injury and therefore it's worth making an effort to do it the right way, but sometimes the wrong way will only be marginally less effective than the right way and in that case it's better to do what you can manage than nothing at all.

In your case I would hold on because it's better to do that than risk falling and injuring yourself. Getting injured is a pain in the neck if it stops you from exercising at all because you'll lose muscle tone while resting and recovering. Maybe, if you feel that it's slightly less effective than not holding on, do an extra bit of time or add on a few minutes of core exercises so you can feel you're not missing out.

Jesus christ 18 incline. How fast are your speed walks? Inspired!

As I said doing something is better than nothing so if you need to hold on when fast walking on an incline then do that. It’s great you are trying to improve your balance too. It’s all going to lead to a healthier fitter you. Which is the goal!

Holding on will mean your leg muscles are doing less work because your arms are taking part of the strain. However, holding on is better than doing nothing!

Doing something is way better than doing nothing. But as we age balance is really important. So if you can I would really try to improve your balance. Not necessarily on the treadmill yet though. Obviously if your balance isn’t great due to medical issues ignore me but there are ways to improve balance. Standing on one leg for instance.

And you could try basic normal walking at a slow speed in the treadmill at the beginning of your workout and not hold on and try to get used to it that way.

I'm really pretty fit (weight lifting etc) and I use the handles on my 18 gradient incline speed walks. This means I work my upper arms as well as the (minimally) reduced benefit to glutes of the incline. Yes, I could work core better by letting go, but it's figuring out your own aims and saying to heck with everyone else.

You probably won’t get as much benefit holding on, but you’ll get even less benefit if you feel too unsafe and stop doing it completely.

The thing with exercise is we all have visions of what we should be doing that don’t always match the reality. So in my case as a wrong side of 50 and overweight lady, I figure as long as I keep moving and doing what I can, safely, it’s all good. If things are too hard or you don’t feel comfortable doing it, human nature is you’ll stop. It sounds like you’re doing well.