Is Prince Harry heir to the throne? Will he be king?

Where he stands

Harry is currently 5th in the line of succession, behind Prince William and William's 3 children.

The fact that Harry and Meghan Markle ceased to be working members of the Royal Family in 2020 did not remove Harry from the line of succession. His position derives from his birth and the constitutional rules governing the Crown.

Why he'll almost certainly never be king

For Harry to become king, all 4 people ahead of him would have to die or be disqualified without heirs of their own. William is healthy, and his 3 children are young. The line is robust and goes several positions deep beyond them too.

Harry and Meghan relocated to the UK with their children, without plans to resume public duties as working members of the royal family. So even the ceremonial proximity to the institution has diminished. He holds a place in the succession on paper, but the realistic path to the throne simply doesn't exist.

What being "in the line" actually means

The line of succession is a legally defined sequence, not a family pecking order you move up by ambition or performance. It is determined by descent and several constitutional and legislative rules. Stepping back from royal duties, moving to another country, writing a memoir that upsets the Buckingham Palace, none of it removes you from the list. Only death, disqualification or an Act of Parliament does that.

His children, Prince Archie and Princess Lilibet, hold the 6th and 7th spots respectively, which tells you something about how remote the whole scenario is.