This doesn't match the guidelines at https://meta.wikimedia.org/wiki/MassMessage as they have been in place since January 2014: "Optionally, add a link to your user page or user talk page on Meta, to mimic a normal signature:..."(my bolding; @Whatamidoing-WMF added that back then "for 'personal' messages")

I agree it's important to give readers a feedback channel, but the Meta user page of the user operating MassMessage might often be the wrong link for that. Frequently users post on behalf of others (e.g. LivingBot every week), and in case of newsletters there should already be a link back to an "About" like page.

How may we require that a reasonable signature (in "person/entity (a link to a monitored talk page) TIMESTAMP" format) is always present?

The hidden comment is added by the "massmessage-hidden-comment" message, so local wikis have the option of making it non-hidden and visible. It's mainly intended as an audit trail rather than information for the user, which should be included by the sender.

In T98662#1274164, @Gryllida wrote:

How may we require that a reasonable signature (in "person/entity (a link to a monitored talk page) TIMESTAMP" format) is always present?

I think this needs to be done as a social requirement rather than implemented by software. Right now MassMessage will warn you if it doesn't detect a timestamp, but that's overridable since it's not 100% perfect.

The software should have a canned response or a tick box to include a visible reader-facing signature.

This should be visible to the readers, not just the people who click 'edit'. Please include it in the markup in a way different from a hidden comment.

Why? I don't see a rationale provided in this task.

In T98662#1274164, @Gryllida wrote:

How may we require that a reasonable signature (in "person/entity (a link to a monitored talk page) TIMESTAMP" format) is always present?

By using discussion software that auto-includes this information such as LiquidThreads or Flow?

Maybe you're bothered by the use of "MediaWiki message delivery" instead of the underlying sender's username? That's tracked at T71954, which you're already aware of.

I don't think this is needed - there are many reasons why senders name is not always needed (for example, Wikiproject newsletters or another group). I think the existing implementation (where the bot timestamp is posted, which is the posing user, and the sending user is included in a timestamp) is better - if users want to include their signature in the message, that's also easy enough to do with the existing implementation (sign the message with ~~~, then append ~~~~~ to the message as you send it).

I think that a visible datestamp is important because of the way archive bots (don't) work. I'm ambivalent about a link to the username: Something like "From WikiProject ____" or "On behalf of ____ Committee" might be appropriate in some cases.

On the other hand, dates are important. Archive bots don't work without them. It's all very well and good to say "just append ~~~~~", but humans sometimes forget to do things that are "easy". I've had to clean up after some of those accidents (none caused by me).

At a minimum, it would be appropriate for the software to notice that a probable user error is about to happen, and to produce an error message about it ("Are you sure you want to have to go around to these 800 pages and add a timestamp manually?").

I join the voices who think that "Posted by MediaWiki message delivery on behalf of the Wikimedia Foundation" signatures feel like lack of human accountability.

Senders can already sign their message. I guess what is wanted here is either warning or enforcement if that is not done.