On the original issue by @tonygrue:

Currently, we don't flatten/render remarkup in commit messages. We have the capability to render remarkup into plain text (as of very recently) but it is nascent and doesn't yet address all the rules. We'd need to improve this before we could really do something reasonable with this feature, although such improvements are on the roadmap anyway (in order to improve plain text email).

Flattening remarkup in arc land decreases the utility of Diffusion, which renders remarkup in commit messages. (If we flatten {T123}, it will no longer embed; if we flatten tables they will no longer display nicely, if we flatten <hr> and headers they won't present properly, etc), which is why we don't do it. That said, I think it's not completely unreasonable to add a flag which causes arc land to render remarkup into text mode for installs that don't like Diffusion.

A concrete problem with expanding Fixes T123 into Fixes T123 Task Tee One Two Three is that the Fixes syntax accepts multiple arguments, so a task named T124, T125 would trigger all of T123, T124 and T125 to close. Similarly, Fixes T123, T124, T125 would become unparseable after expansion without extra escaping, and trigger only T123 to close. So we would either need to expand with escaping (which sort of defeats the original point?) or expand as a footnote (which maybe also defeats the original point?). This also makes expansion more difficult in general, because we need to capture the Fixes syntax as a remarkup rule when rendering.

So, while this is plausible, it's really complicated, involves a bunch of tradeoffs, and we end up with an optional feature of debatable value. We'll move closer to being able to implement it anyway, but I'd like to see broader demand for it before building it.