In actual fact, Yadif mode was present in 2014, so maybe version I have is updated compared to Sharc's version.
I'm guessin that Slower is maybe QTGMC, but cannot find working manual as yet, site is down.
EDIT: Current docs seem to be incomplete:-
https://handbrake.fr/docs/en/latest/...s-summary.html
EDIT: There are several HandBrake alternatives (GUI's which use the CLI version of HandBrake), eg EasyBrake,and VidCoder.
EDIT: HandBrake Forum:-
https://forum.handbrake.fr/
EDIT: Suggest search on above forum, with eg "Deinterlace,Slower" or whatever valid options are available in your version HandBrake,
eg "Deinterlace,Yadif" for old version about 2014. Forum seems quite active.