Has anybody ever heard of a modern composition notation convention where sloping beams indicate acc. and rall.? A composer colleague of mine who always uses flat horizontal beams claims this (without any special notes in the prefaces to the music). Is there any documentary evidence for such a thing? Do post WWII musicians really read this extra semantic layer into beaming as a matter of course and standard practice? I confess to being very surprised to hear this. I am not referring to modern feathered beams, which are used for acc. and rall.
Tilting beams to indicate changing tempo is not standard practice.
It might be warranted for a composition with a plethora of gradual tempo changes, but it discards the many well thought out rules for beaming (dozens of pages in Gardner Read's "Music Notation"), and thus harms readability. For a piece with that many tempo changes, better to renotate it as individual note durations in a constant tempo.