Normally, yes, but not always. There's evidence on the Wiki page for each world having previously open migration routes (possibly for facilitating shuffling accounts about when a bunch of the numbered worlds were closing) when migrations were being utilised and worlds were being managed properly.
As a couple of examples; Arizona had migrations to W11, W12 and Briscoe
, Colorado had migrations to W1, W11, W12, Arizona and Briscoe
Also the Beta worlds have just had an open migration policy since migrations were introduced, but that's slightly different.
There's also examples of worlds that players didn't want closed, like World 1, which was forced to close and players migrate - although it lived on for a couple of years after signups to that world were closed and half of them had already regrettably migrated out.
I wonder if it's possible, even, that the negative feedback from closing worlds players didn't want closed, is why we presently have 13 Worlds - which is around what we had, when the game was at its absolute peak, and with a fraction of the player-base to support it.
I'm not goober and maybe he has a different take, but "Optional migrations" is an attempted compromise to a problem where we aren't really sure what the problem is. We don't know why a world is closed, or why it's decreed to be active enough to stay open.
Basically it just means letting the players who are seeking duels, forts, market activity, or chat - the stuff that this game was traditionally about - migrate to a world that has that. What is clearly a graveyard world to those players, still has enough clicks-per-second, nuggets-per-tombola, or whatever unknown metric is used to measure activity, to be considered active - so let them happily click away, but it's only fair to let the others escape to find a new adventure with their character.