A simple custom module can resolve this.

Versions - Odoo 16.0, 17.0, 18.0 and 19.0

Layout:

web_style
    __init__.py 
    __manifest__.py
    static/static/src/css/style.css

Contents:

__init__.py

(empty)

__manifest__.py

{
    'name': "Web Style: Underline Fields",
    'depends': ['web'],
    'license': 'LGPL-3',
    'assets': {
        'web.assets_backend': [
            'web_style/static/src/css/style.css',
        ],
    },
}

static/static/src/css/style.css

/* Add a soft underline to all fields in the form view */
.o_form_view:not(.o_field_highlight) 
.o_field_widget:not(.o_field_invalid):not(.o_field_highlight) 
.o_input:not(:hover):not(:focus) {
    --o-input-border-color: var(--o-input-border-color-default, #E7E9ED);

Result: