While Flash is no longer common, there are still some heavily-used sites that require Flash. For instance, many chemistry departments use Pearson's Mastering Chemistry for student homeworks, and this site requires Flash.
Mastering supports the way chemistry students learn best: through engaging, immersive experiences.
www.pearsonmylabandmastering.com
I don't know why they haven't switched to HTML5—perhaps doing so would be a big project for them, and they haven't wanted to invest the resources.
Given this, I don't think Apple's decision will get such sites to move away from Flash any quicker than they otherwise would, because the coders for these sites have already known about the movement away from Flash for years.
What will force them to switch away from Flash is, instead, Adobe's decision to kill the software by the end of 2020.