Translation:He is still very popular today.
I would take "encore très populaire" to mean "still very popular", and "très populaire encore" to mean "very popular again". The placement of "encore" (before or after "populaire") changes its meaning. Another example: "J'ai encore faim" = "I'm still hungry", "J'ai faim encore" = "I'm hungry again".
"J'ai encore faim" can mean either "I'm still hungry" or "I'm hungry again".
But in our sentence, as native speaker, I would say:
- Il est encore très populaire aujourd'hui. <-> He is still very popular today.
- Il est à nouveau très populaire aujourd'hui. <-> He is again very popular today.
At first, I thought it was due to the "aujourd'hui" but in fact even without it, I wouldn't get it as "again" if someone said Il est encore très populaire., even in a context where the person was popular then wasn't anymore up to now.
And placement doesn't change anything for me.
Sorry, I don't have a source to refer to, it's just how I understand the sentence as native speaker.
I think that it's because you're rarely popular then unpopular then popular then unpopular etc... So "again" implying (almost always that it happened before several time), it would be odd to use it there.