in Odoo 18 we can do both path and using fields also.
I already tried this approach but it's no working

Hii,
Check This:
Open a sale order in Odoo

Enable Developer Mode

Click the bug icon → "Edit View: Form"

Note the External ID — is it really sale_subscription.sale_subscription_order_view_form?

If not — you are patching the wrong view.
start_date and end_date are coming from another model
Use Developer Mode to check the field origin:

Hover over the field label → if it's a related field (e.g., from subscription_id.start_date), you must modify that model/view, or set readonly in the Python field definition.
Some view or module is overriding your change after yours loads
To rule this out:

Give your view a high sequence:
<field name="priority" eval="1000"/>

OR place it at the bottom of the data list in your __manifest__.py

Try This Final Setup (Full Example):

<record id="sale_order_start_end_readonly" model="ir.ui.view">
<field name="name">sale.order.readonly.start_end</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale_subscription.sale_subscription_order_view_form"/>
<field name="priority" eval="1000"/>
<field name="arch" type="xml">
<xpath expr="//field[@name='start_date']" position="attributes">
<attribute name="readonly">1</attribute>
</xpath>
<xpath expr="//field[@name='end_date']" position="attributes">
<attribute name="readonly">1</attribute>
</xpath>
</field>
</record>
i hope it is usefull
Add this temporarily to verify:
<xpath expr="//field[@name='start_date']" position="replace">
<field name="start_date" readonly="1"/>
</xpath>

I did this, The external id is sale.view_order_form but there is no field called start_date and end_date.
so I check the inherited views and then find it is subscription module's view and tried to change the attribute but it is not working.
I did it using python field as well but don't know what wrong with this!