Doesn’t seem domain specific to me.  Instead it answers whether a text has any emphasis or not.  By definition strings don’t.

_,,,^..^,,,_ (phone)

> On Mar 27, 2023, at 1:01 AM, commits at source.squeak.org wrote:
> 
> Marcel Taeumel uploaded a new version of System to project The Trunk:
> http://source.squeak.org/trunk/System-mt.1403.mcz
> 
> ==================== Summary ====================
> 
> Name: System-mt.1403
> Author: mt
> Time: 27 March 2023, 10:00:56.498651 am
> UUID: 106cd43c-6be0-314c-ac46-e16bf55f8a89
> Ancestors: System-eem.1402
> 
> As usual, move domain-specific string/text messages into extension protocol. Use *System-Support for now but it could be *Network as well or something that relates to the underlying MIME classification (here: text/plain).
> 
> =============== Diff against System-eem.1402 ===============
> 
> Item was added:
> + ----- Method: String>>isPlainText (in category '*System-Support') -----
> + isPlainText
> +    "For compatibility with Text"
> +    ^true!
> 
> Item was added:
> + ----- Method: Text>>isPlainText (in category '*System-Support') -----
> + isPlainText
> +    "A Text with no emphasis has empty values."
> +    ^runs values = #(#())!
> 
>