Those who missed the 20 years of the show, this is a very rare footage.
‘For’ those who missed the 20 years of the show, this is a very rare footage.
Does the ‘for’ in the second sentence sound odd or not?
Those who missed the 20 years of the show, this is a very rare footage.
‘For’ those who missed the 20 years of the show, this is a very rare footage.
Does the ‘for’ in the second sentence sound odd or not?
No, actually. In fact, the "for" in the beginning sounds very natural, and better than missing it out.
It would make sense to skip the "for" if you're saying something like "those who missed the 20 years of the show have been sitting under a rock".
"For those" indicates something more like a disclaimer or message to the people who missed the 20 years of the show. So for makes more sense in this case, like: "For those who didn't watch the 20 years of the show, this is very rare footage."