Not a stretching exercise but a technique that seems to work. We have a number of 65 yo and up (3 in their 80s) cyclist in a group I ride with. Instead of swinging their leg to the back and over the saddle, they swing their leg forward, bending the knee and come over the top bar. With very little practice it works very well and you save lifting your leg the additional 6-8 inches or more with the traditional dismount. It actually looks pretty cool and whatever works...
OK, but there is no reason not to strengthen your hips and increase your flexibility so that you can do it just like you used to do it, unless there is some physical reason such as arthritis.
Why modify things when a bit of stretching and strengthening keeeps things like new?
I have more lfexibility and strength than a lot of 40 year olds.