A great deal of modern practice is organised around sequence. Postures are linked, orders are fixed, and in some methods the order is the method. The classical manuals contain almost nothing of the kind. Their posture chapters are lists: an entry, a description, sometimes a claim, then the next entry, with no statement about order and no account of how a practitioner moves from one to another.
The three or four places where a text does connect two postures therefore carry a lot of weight, and uttana kurmasana is the clearest of them. The Gheranda Samhita has two more, matsyasana entered from the bound lotus and uttana mandukasana entered from the frog. That is the whole of it.
Mark Singleton's Yoga Body documents where linked sequencing came from, and the answer is largely the twentieth century, in the physical culture environment in which modern posture practice was assembled. The atlas records that the classical corpus attests a very small number of transitions and does not attest sequences, which is a statement about the texts rather than about what practitioners in any period actually did. The manuals are terse and were written for people who had teachers, so silence in them is evidence about the record, not proof about the practice, and the entry says so rather than overreaching.