That is what they did. The number of bits is a detail. There is no way to use a different number of bits and remain compatible.
Of course you probably never worked with source routing in IPv4 - I spent a lot of time testing it because marketing couldn't prove none of our customers used it. Even then nearly all routers shipped configured to drop any packet with source address routing, but to comply with the rfcs we had to make it work.
The only place where source routing is used these days is MPLS-SR aka Segment Routing. The edge router precompute the path and sets all labels. Core routers just pop those labels and switch.
Most of time you just use normal routing or ocassionaly PBR (Policy Based Routing) with is not source routing. Src IP can be one of the parameters to decide.