Is the throwing away of leftover food considered blameworthy extravagance (israf makruh) rather than forbidden squandering (tabdhir muharram), or is there a consensus on that? And what is the ruling on extravagance in leaving the water tap running to facilitate washing dishes, wudu, and ghusl, knowing that reducing the flow would take longer?

If the food remnants are usable, it is not permissible to discard them, as this constitutes squandering money and forbidden extravagance. Rather, they must be given in charity to those in need, as stated in the fatwas of the Permanent Committee and the saying of Sheikh Ibn Uthaymeen. However, if they are not usable, there is no harm in discarding them.

Furthermore, leaving the faucet running for longer than necessary is a disliked form of extravagance according to many scholars, and forbidden according to some. A Muslim must avoid extravagance and wastefulness, and preserve Allah's blessings.