Most mods nowadays don't need to touch minecraft.jar at all. Forge does, but it provides hooks for other mods to do most anything they need to without actually messing with vanilla Minecraft code. There are a few mods that do want to do things that Forge does not provide hooks for, but they can handle it via ASM- hacking into the Java bytecode in memory- without actually changing anything in the jar files on disk.
In short, if support for jar mods (other than Forge) has been dropped from the FTB launcher, it's because it's really not needed anymore.
However, there likely are still a few modders out there who still make jar mods (such as Optifine, maybe?), and if you really need to use such a mod, you can install it the old-fashioned way: by going into the appropriate FTB instance folder, opening up minecraft.jar, dumping in the contents of the mod in question, and deleting meta-inf.
Also note that a "jar mod", as I use the term, is not the same thing as a mod distributed as a .jar file. Things like "ThermalExpansion-[1.7.10]4.0.1-182.jar" just get dropped in the instance's mods folder wholesale, not opened up and dumped into anything.