Do we know if the 10 user limit for free tier includes all former users of a board?

I'm using a Trello board to manage some basic tasks for a nonprofit I'm working with. We only have about eight people who use it, but people will come and go, and recently I found that I couldn't add a new person to the board, even if I deleted many of the old people.

Do we know if the max users is the number who have been on that board at all, even if they're gone, and their cards are deleted?