Hello 

Actually the datatype of the field "removal_date" in stock.production.lot is datetime, and this field is referred as related fields in the stock.quant object.

ex :

class StockProductionLot(models.Model):
    _inherit = 'stock.production.lot'

removal_date = fields.Datetime(string='Removal Date',
help='This is the date on which the goods with this Serial Number should be removed from the stock.')

class StockQuant(models.Model):
    _inherit = 'stock.quant'

    removal_date = fields.Datetime(related='lot_id.removal_date', store=True)

here the issue is you are changing the datatype from datetime to date in stock.production.lot, but not in stock.quant thats the issue,

if you include this change your code will work

class StockQuant(models.Model): 
    _inherit = 'stock.quant' 

    removal_date = fields.Date(related='lot_id.removal_date', store=True)

Thanks