For now it's completely free for the individual accounts (education accounts have a small fee: https://flat.io/edu/pricing).
The individual ToS are available here: https://flat.io/legal/tos-individual
(...) 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.I understand that's not how you use it just now. But that's what it says.
I can see why artists might be a little concerned about this.