I have been trying to manually add Font-Awesome 5.1.1 with my phpbb 3.22 installation. But something goes wrong. Maybe that's because in font awesome 5.1.1 they have separated the fonts like All,Solid,Brand and they also provide different CSS, Webfont file for each category. And in FA 4.7.0 which is currently default installed by the phpbb 3.2.2 have only one css and one webfont file.
So anyone can help me how to figure out this issue. Or proper guidance on how to update FA manually?
font-awsome 5 is not compatible for the reason you mention. It will not be added to the core unless they have solved the issues with the pro version releasability If they solve it at all. In future versions, we will be overhauling the entire icon system to allow for more flexibility/control and simplicity. To use svgs/fonts/png based icons. We will also be switching to material svg icons as the defaults.