Hi E.M.

First of all, why did you inherit an odoo view defined with the record tag with the template tag? it's not the same, in the record tag you have the freedom to set record field values like model and others that cannot be set while using the template tag. The template tag is a wrapper around a very basic ir.ui.view qweb record without been tied to any model, so when odoo view inheritance search for the view inherits of the same model will not find your template as an inheriting view

You could get it working like:

<record id="placeholder_view_partner_form" model="ir.ui.view">
    <field name="name">res.partner.form</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="base.view_partner_form"/>
    <field name="arch" type="xml">
        <xpath expr="//field[@name='website']" position="replace">
            <field name="website" widget="url" placeholder="e.g. www.foo.com"/>
        </xpath>
    </field>
</record>

or a better way

<record id="placeholder_view_partner_form" model="ir.ui.view">
    <field name="name">res.partner.form</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="base.view_partner_form"/>
    <field name="arch" type="xml">
        <field name="website" position="attributes">
            <attribute name="placeholder">e.g. www.foo.com</attribute>
        </field>
    </field>
</record>