electron9 was dropped from the Arch repos yesterday. I added it back to the AUR. Compilation will take a long time (and a lot of RAM) as you're essentially compiling Chromium.
electron9-bin is in the AUR (as used by Manjaro for their electron9 package) if you need it - though it was removed from the Arch repos because nothing in the Arch repos uses it (so you probably don't need it).