I have an expansive modpack with the whole RPG Series modpack and Arsenal, which completely overrides the Simply Swords loot table. Runic tablets still manage to spawn, however, upon killing the wither for the 13th time, with modified spawning percentage, not a single standard/rare/unique Simply Sword has ever been dropped/looted even from chests. I believe this might have something to do with both mods conflicting and serving a very similar purpose.

This issue has been reported, and has already been fixed on github, but hasnt been released yet. The next hotfix will fix this issue.
The issue is actually an issue with all SimplySwords weapons not spawning, and running /reload whenever you open the world should fix it for now.

The next update will include this fix, though I do not know when Sweeney will have time to do that.

Fix is now live ^