The reason to disable attachment may be anything like
1. There is no actual need from client for attachment
2. Server Space constraint/restriction
3. Avoiding malicious file upload (security reson - VAPT observation) etc.,
To remove add attachment option from chatter module
File : /<your_path_to_odoo>/addons/portal/static/src/xml/portal_chatter.xml
Comment the following line: (In odoo14 - to be precise at line no - 55 )
<button class="o_portal_chatter_attachment_btn btn btn-secondary" type="button" title="Add attachment">
<i class="fa fa-paperclip"/>
</button>
<!-- <button class="o_portal_chatter_attachment_btn btn btn-secondary" type="button" title="Add attachment">
<i class="fa fa-paperclip"/>
</button> -->
For your reference and perusal screenshots before and after the code block comment.
