[SalesForce] Read only field – Still editable

I know you said no validation rules in your question, but based on your comments it seems like it's more of the aesthetics that's keeping you away from them. From personal experience, as long as you have a good error explanation that doesn't leave users confused, they have no problem with validation rules. In case you decide to go this route, here's a rule that may work for you.

AND($Profile.Name = 'Read-Only Profile', ISCHANGED(Lead_Status))

Error Message: Lead Status is a read-only field for users with the profile 'Read-Only Profile'

Hope this helps if you decide to use this.

Are you doing this as a system admin?

System Admin profile has the option "Edit Read Only Fields" under "Administrative Permissions" and this means that profile is always able to edit read only fields.

Try logging in as a user with a different profile.