Just found the TOS while browsing.
The link under "sign up" (leading to
https://flat.io/legal) was the one I referred to as broken.
(...) you grant us a worldwide, non-exclusive,
royalty-free and perpetual licence to use,
copy, reproduce, distribute, adapt, re-format,
modify, publish, translate, licence, sub-licence,
and exploit the User Content anywhere and in any
form for the purposes of providing our Service (...)
Yeah, I'm sticking to Lilypond, thanks.
