If I try to check what the feedrate is by the control tab, I get the info that "The feedrate can only be set; it cannot be read back due to a limitation in the communication protocol." However, typing an M220 into the gcode terminal outputs the current feedrate. So clearly it is possible to read the feed rate.
I couldn't find a plugin that polls feedrate from M220 and puts it into the GUI. How do I do this?
Do you mean how you develop a plugin?
I don't necessarily want to write a plugin unless it is the easiest solution; I only want to get the feed rate to be polled from the printer to the OctoPrint control tab. Scripts, settings, etc. are also acceptable.
I'm not aware of any way to get the feedrate right now (in the UI).
So I guess the easiest way would be to write a small plugin for this job.
If you need help you should visit us in the OctoPrint Discord and talk to other plugin devs 
edit: fyi here's the github issue where this feature was discussed [Request] Feed and Flow rate is readable (from Marlin 1.9) · Issue #4013 · OctoPrint/OctoPrint · GitHub