Before this thread closes I thought I might add a word about the same problem of pacman refusing update because of “icu” causing dependency conflicts. Also to thank you @ddnn for your help in the Wiki about pacman updates being blocked.
The very useful command yay -Sii <pkgname> gives a whole raft of useful info about where and when a package was installed as well as dependencies.
In fact I came across it while searching for conflicts caused by qt5-webkit and the pactree command showed me that it was another AUR package I had installed that required it. By uninstalling the conflict-causing packages I was finally able to do a pacman update successfully.
Thanks again for your helpful Wiki. 