My first horse was a trail horse and one that needed a job. We just dialed back on the rides length and intensity as he let me know what he was happy doing on any given day. I rode him regularly right up to the end.
My last horse retired from competition at 12. He had a fixed, inoperable bone chip in his hock. We had to do a lot of stifle strength exercises as he used his stifles to compensate for the bad hock. In his second last summer it was becoming too much unpleasant work and I decided that was enough. Even if it meant hastening his end. I wanted him happy.
He was the sort of horse that could be content living out in his field and coming in for food and grooming. Riding slipped away gradually. I had other horses to ride seriously, and it was okay if I only had him out for a hack 3-4 times a month. He enjoyed regular grooming, and was content.
The older of the two I have now has some health issues that preclude that sort of field retirement. When I stop riding him he will go quickly. But I will listen when he tells me he’s had enough. Even if it hastens his end.
I think it is less a step than a gradual slope as you find yourself doing less demanding things over time. And both of you are okay with that.