If restrictionist is someone who wants restrictions on immigration, what do you call someone who opposes same?
Generally speaking, people on the opposite end of the political spectrum from immigration restrictions are known as open border advocates (or something to do with "open borders")
I’m talking about opening up America’s borders to everyone who wants to move here.
Imagine not just opposing President Trump’s wall but also opposing the nation’s cruel and expensive immigration and border-security apparatus in its entirety. Imagine radically shifting our stance toward outsiders from one of suspicion to one of warm embrace. Imagine that if you passed a minimal background check, you’d be free to live, work, pay taxes and die in the United States. Imagine moving from Nigeria to Nebraska as freely as one might move from Massachusetts to Maine.
Additionally, the term "antirestrictionist" can also be used. For example, in The Dilemma of American Immigration: Beyond the Golden Door, Cafferty et. al describe their arguments as practical rather than philosophical:
We do not wrestle with issues of immigration in a theoretical vacuum of philosophical debate between "restrictionism" and "antirestrictionism;" but in the practical order, where there are concrete political, social, and economic problems for both the United States and the sending countries.
Western Hemisphere Immigration and United States Foreign Policy edited by Christopher Mitchell has a whole chapter on "Domestic sources of Antirestrictionism".