Somebody suggested (Efficiency VI) on my previous Wither skeleton farm video that I use a portal to move them to the overworld. Originally I had built it in 1.5.2 and did toy with the idea at the time but getting the Wither skeletons through the portal would require a laggy piston set up or an afk position above the portal to lure them in. I didn't particularly like either solution so I didn't bother.
Anyway, with the recent suggestion and the now bigger portals I revised the farm slightly (it still works by letting unwanted mobs despawn where possible but seperating mobs in the overworld is a lot simpler).
I linked five portals together with a sixth used for player access (toggleable to stop pigmen gathering inside when AFKing for long periods)
AFKing for one hour in perfect conditions will yield approximately 750-800+ Wither Skeletons. I also made a much smaller version meant for AFKing overnight which will produce around 100 Wither skeletons per hour.
I did look on YouTube and only found one other design which was uploaded recently. It was basically JL and Kabo's WitherRipper with a portal add-on which works well also.