AWS must be able to delete the data but since you stopped paying it must not be important to you anymore. No one outside of AWS can delete the data and presumably no one inside AWS can delete Object Locked data IF your bill is payed.
I'm almost sure S3 keeps data and eats the cost. Although, my understanding is from 7 years ago when I worked on the team that would have implemented it.
If they do, then it probably would require a human to run a tool. It would have to be big bill to warrant that.
Pure speculation but the semantics of and usual purpose of “legal hold” strongly imply that keeping the data even with an unpaid bill would be the correct implementation. Even at much smaller scales contracts state fixed data retention or escrow schemes beyond the end of the contract precisely to make legal discovery easier
Edit: went looking for some evidence that supported this (at least in the stricter compliance mode) but found none in the docs. I found a 3rd party audit report supporting the use of this mode for SEC/FINRA etc regulated data and it says the entity is still responsible for maintaining its account in good standing but goes into no further details on what AWS does if you don’t.