Numeric fields such as IntegerField, FloatField and DoubleField support a "isReadOnly" property to prevent modification.
While this correctly prevents the user from modifying the value with the keyboard, she can use her mouse to modify the value (try to drag the table and see your cursor change).

Steps to reproduce:
1) Drag attached script inside a project
2) From the top menu, open "UI Toolkit > ReadOnlyField"
3) Drag any of the readonly field

Expected result: values are not modified
Actual result: values are modified

Occurs with 2019.4, 2020.3, 2021.1 (not verified by report on the forums), 2021.2.

Forum:
https://forum.unity.com/threads/numeric-readonly-field-can-be-modified-by-dragging-label.1110823/