I'm no expert myself but recently had a relative in hospice,[0] which prompted me to look into this issue a bit. There are a few papers (for a recent example, Kingdon et al. (2020))[1] that say that the data is actually extremely low-quality on both sides of the issue. Personally, this makes me wonder if "fluids are bad" might just be received wisdom and/or anecdata... hopefully someone will do a better study(ies) at some point in order to get actually reliable data.

[0] We did end up following the hospice's recommendations not to administer IV fluids, for what it's worth.

[1] https://spcare.bmj.com/content/bmjspcare/11/1/68.full.pdf