Understanding Censorship resistance in POW

With the upcoming transition of another unnamed network to Proof of Stake there has been a lot of discussion around transaction censorship.

Can someone help me with a description of what the risks are within Proof of Work with regards to censorship? What are the key mechanisms that will keep censorship at bay if top miners and pools decide to start censoring transactions? Is it a cut and dry 50% of miners must be publishing all blocks? Or if 90% of the blocks are coming out censored does it fall to user nodes to continue validating those transactions?

Comments and resources appreciated. Thanks!