There are some chord progressions and stuff where they took inspiration from other artists, and they've been reluctant to admit it as it could entail giving up songwriting credit for a song they obviously made themselves.

It's a stupid byproduct of how the music industry has fucked up music. It was always based on borrowing and reimagening the music of others, especially the folk and blues traditions Zeppelin found themselves in.

Guthrie's copyright notice is one of the few respectable ones:

This song is Copyrighted in U.S., under Seal of Copyright # 154085, for a period of 28 years, and anybody caught singin it without our permission, will be mighty good friends of ourn, cause we don't give a dern. Publish it. Write it. Sing it. Swing to it. Yodel it. We wrote it, that's all we wanted to do.

Here's a BBC article about the Stairway copyright dispute. And here's the song in question, with the allegedly stolen part starting around 0:45. It's likely that Zeppelin took inspiration, but also it's a completely different and way less interesting piece of music. Great example of how copyright is used to kill art and creativity rather than to protect it.