Miro currently has a range of shapes users can draw, but the choice of shapes offered isn't necessarily the most useful for representational purposes.
The default triangle you can draw in Miro is an isosceles triangle. It is incredibly challenging to get this to form right-angled triangles when needed.
To give an example, I'm drawing a timeline where I want to represent a 'phased transition between two states over time'. The most convenient visual representation would be a diagonally bisected rectangle with two distinct areas of different colours.
Currently there is no ready way for me to do this in Miro without expending a lot of effort fiddling with the isosceles - to very limited effect. There isn't even really a sensible workaround for this within Miro's current feature set, and its definitely a limitation.
As a feature, right-angled triangle would be far more generally representationally useful (I'd argue perhaps even more than the isosceles) than some of the other 'basic' shapes - in particular the parallelogram (which is awkward as hell) or the star (which is useful as an icon, but has less 'compositional' use than a right-angled triangle).