I’m getting shiny/reflective artifacts from tree leaves textures that are reflecting sunlight and making them pop out, but only when the particular texture is viewed from an extreme angle (thus squishing the texture down). It seems like a specular thing, but I turned specular all the way down and roughness all the way up (I’m using Blender). Is there anything else I can do to prevent this visual issue?
Did you maybe try using a different texture (black, or some other darker colour) - to see if it’s an issue with the material or the texture itself 
?
Your textures seem to have a bit of a white outline (judging by the branches on the left and lower part of the tree.)
It may be possible that the “shininess” is caused by texture filtering / anisotropy levels visible at sharper angles (I keep forgetting which is which, so here’s a little explanation, and also the best gif ever.)
The texture gets blurred as the surface of the plane becomes smaller due to a sharp angle, so white artefacts at the edges could start to influence the green colour on the rest of the texture - and give a fake “shininess” effect.
thanks for the tips, I’ll look into those links!