I understand that the Ashkenaz and Sephardim sit and stand at different times. I came across a footnote that the Ashkenaz stand during the daily morning reading of Psalm 100 and The Sephardim sit during the reading of Psalm 100. Is this still the case or has this particular distinction ceased to exist?

Some stand for this paragraph ("Mizmor l'Todah") while others sit, and both customs are acceptable and legitimate. My grandfather z"l, even when he was in his mid-90's was careful to make the effort to stand up for those several seconds in order to say this short Psalm, because that was/is the predominant custom in America. The issue is not necessarily "divided" by ashkenazim or sefaradim lines. Just if your community stands, you clearly should do so as well.