The title has been changed from the original title, because that is what I meant, but I misused the terminology, and probably some physics, asking about evidence for the existence of a horizon for the observable universe?
Though my question was a mess, Christoph put some work into giving it a cogent answer, clarifying some points, pointing out errors, and adapting to what I really wanted, which I explained in a comment. As I was asking him, he suggested however that I do not change much the question, which contains probably common misconceptions.
With my thanks to him, here is the question as originally asked:
From what I understand, this observable universe horizon results from the expansion, so that when galaxies are sufficiently far away, the expansion speed relative to us is faster than the speed of light. That is clean and simple reasonning.
But, isn't it possible to imagine a geometric structure of the universe such that the speed of light is never exceeded despite the expansion. Then there would not be an observation horizon.
After all, we found with relativty that speeds add in a more complex way that the simple addition we were used to. Could it be that the simple expansion analysis that we use to justify the existence of a horizon for the observable universe could be refined and give a different result?
What we observe is not very far (since the universe is only 13,7 Glyr old). More precisely, we observe it from a time when it was not so far (sorry for the over-simplified statement), even though what we may observe is today much further away. But there is no way to observe things that approach the suggested distance of the horizon (around 80 to 90 Glyr, from various posts).