I am recording the steps I took here to enable hibernation on a default Pop!_OS installation with full-disk encryption. I have written my commands as a single executable script in order to better understand how they all relate, but they probably should not be executed this way. Use this guide with caution and only if you understand what each command does.
I did all of this on Pop!_OS version 21.04, but swapfile hibernation is still working for me as of 22.04. I expect that the following steps will work for that as well, but I haven't tested it.
This is the simplest method if you used the default Pop installation, since it does not involve resizing any encrypted partitions.