He had a solid year, showing some decent stats as a true #1 WR. I doubt he would have anything notably better had he stayed for 2019. There's no sense in risking injury, as he was able to finish this year without any significant injuries, and sometimes, it's better not to tempt fate.
Right now, the knock on him is his sheer speed, where he probably runs closer to a 4.60. He would fit the mold of "strong possession receiver" at the NFL level, which isn't very glamorous, nor are they drafted particularly early. Even if he had a better year in 2019, I doubt it would change his predicted draft position as a late round pick.
There's a good bit of younger talent coming through the ranks, too. Now that Kevin Austin will have had an entire off-season, I'm pretty sure he'll be ready to step in, and that would have taken away some reps from Boykin.