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I'd like to use wood for some bean and cucumber trellises but I'd also like it to not decay so easily. So would pressure treated wood leak into the soil? Or am I just going to have to go with plain easily decayable wood?

possibly

consider starting with untreated lumber from a more decay-resistant wood (cedar, cypress, locust, etc)

They used to always advise against using pressure treated woods for raised beds, because they used to use arsenic in the pressure treatment. Since then, they changed the forumulation and took the arsenic out and there's a lot less concern about it. But that was talking about raised bed timbers anyway, where the whole inside surface would be buried in soil. For your trellises, only the little legs will be in the dirt, so I don't think there's any concern. If you really wanted to be super purist, you could wrap the legs where they go into the soil with foil or plastic, but I don't think there's anything to worry about.

But I think a lot of wooden trellises are cedar, which is naturally rot resistant and isn't treated. Or you could get the PVC-vinyl ones that look like painted wood but last for ever. Or metal, like powder-coated steel.

I avoid using the pressure treated stuff in the vegetable garden just on general principles - despite the change in formulation.

was Chromated Copper Arsenate (CCA) but since 200x(?) Amine Copper Quat (ACQ) and Copper Azone (CA) are used.

plain ole' white pine will last 15 yrs+ where it is not left all year round in the soil - which is not normally an issue - in the fall everything gets moved out to some "storage" spot for garden cleanup.

I use white pine 2x4's for my compost 'container' - lasts for a decade or more -and it obviously gets a lot of 'wet contact' - it really shouldn't rot so fast to be a concern - besides, by the time it has rotted out you'll probably have a better idea on how you wanted to build the trellis anyway [g]