How do I find out which plugin is exceeding memory limits?

The error doesn’t say that the plugin is using a lot of RAM. It says that the plugin tried to raise the memory_limit to 256M. These are two different things.

A plugin, or even the core code, trying to adjust the memory limit up very high is fairly normal, if it thinks that it might be doing something that will use a lot of RAM. WordPress does it when doing image processing, for example, because large images need lots of memory to work with, temporarily.

In this case, it appears that the Wordfence plugin has a line in it somewhere that calls that iniSet function to try to raise the memory_limit. That’s it. Doesn’t say that it has actually used any of it, and setting a higher limit is not particularly unusual.