Can "expirable" be a word?

First of all, expirable is a word. It's been hanging around since at least 1913.

expirable

Adjective

That may expire; capable of being brought to an end.

Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing

wiktionary

That said, expirable makes me think it has an expiration date. If that's the impression you want then I think you're fine.

Wish more programmers took this much care when naming things.

Expiring URL vs Expirable URL

The semantic difference here is an Expiring URL had better always have an expiration date. Where an Expirable URL might have it's expiration date set, it might not. So pick the name that goes with how you intend to implement.