I just rent a VPS, temporarily host the file on there, and give whoever it is a link.
Edit: If my threat model were to include protecting the contents of the file from the VPS provider I would probably encrypt it client side with GPG or something similar.
You can still use ffsend for now, it still works and uses the Send instance I'm hosting (send dot vis dot ee). Some generous users started sponsoring me through GitHub, so that'll definitely help keep it alive longer.
First transfer.sh and now Firefox Send are closed. What service should I trust to send file quickly and securely.