I can't say if legal or not, but I can say everyone (30+ people) I talked to while there for the month of Feb 2021 talked openly about spearfishing while diving. I didn't see anyone actually doing it, in tour groups it probably wasn't allowed since they expect you to be following the group closely, but on the shore and boats plenty of people talked about other dives (not tours) where they speared lionfish (I think I got that right), which I guess are really tasty and invasive or something, so they encourage people to kill them.
For your googling you might try looking for info on Caribbean more so than Puerto Rico, if you haven't already (probably have). Most of the laws I have found before, like lobster harvesting, is just general for the entire Caribbean, which I imagine applies to all of PR, or at least anywhere a little off-shore. In general it is hard to find anything specific to PR + Scuba, since it isn't one of the top destinations for Scuba (by far).