It is possible to shoot arrows and Tridents through nether portals.

Here goes your requested formallity:

what I expected:
Arrwors and Tridents get "stuck" in netherportals because they have a hitbox like every other block.

What happend:
Arrows and tridents can pass throu nether portal blocks

steps to recreate:
1. build a Netherportal
2. Grab a bow & arrow and/or a trident
3. hit F3
4. aim for the borders of the block (the black outline shown when aiming at a block)
5, make sure you can change the outlined block by only moving the mouse by one or two pixels.
6. shoot. You should observe that the arrow/trident passes through the portal, insted of getting "stuck" like it would when you aim for the middle of the block.