The field / configuration option you're looking for in Odoo 18.4 now only resides in Accounting -> Configuration -> Settings --> Section Accounting -> Default Accounts:

See also https://github.com/odoo/odoo/commit/6422427ce10dfbe74186985bfe8da540d11cebde

Commit 6422427
igbe-odoo committed on Jun 23 [2025]

[IMP] sale, l10n_fr_account, l10n_be: improve down payment

Move the property_account_downpayment_categ_id field to the company (instead of product.category).
The reason for this is we want to simplify the downpayment accounts. Having one account per product category could lead to downpayment invoices with a lot of lines, as we split lines based on account. Now that we have only 1 account per company, we only split lines based on taxes.

task-4640319