> I don't know how to check if there's a PR open on this

Go to http://www.netbsd.org/Misc/query-pr.html

searching on the text "pfil_list_remove" shows that there is a PR:8296
that looks like it's exactly what you're looking for.
How-to-Repeat shows "ipf -E ; ipf -D ; ipf -E ; ipf -D"

Only if you have defined DIAGNOSTIC in the kernel will it actually
panic in net/pfil.c:
	#ifdef DIAGNOSTIC
		panic("pfil_list_remove");
	#endif

That doesn't make it less of a bug, of course...

> Now the question

Unfortunately, I can't help with this.  :-(

You might try posting to tech-net@netbsd.org instead of the mac68k
list.  If you mention the pfil panic and the PR #, you might be able
to get someone to look at it.  This is supposed to be a "bugfix week".

-allen