From the gitter chat, escape is supposed to abort a history search but it doesn't:

hi all, i've started using fish for a couple of weeks and i find it awesome. however it seems i'm not able to abort history search. the documentation says hitting Esc will abort the search, but nothing happens when i try it. am i doing something wrong? can you help me to fix this? (using fish version 3.0.2, macos 10.14, apple terminal)

Credit to 'mrrech' for spotting this.