This means that, once I prepare new packages (starting in experimental, I think), I will need volunteer testing and feedback, from admins as well as actual users. We are on a very tight timeframe, and I wish to have this all finished within the next 28 days, starting Monday.

I'm willing to help test if it will help get MW 1.23 into Jessie.

Failure to do so will mean shipping Mediawiki 1.19 in jessie, which is currently upstream’s oldstable and fading LTS. Mediawiki 1.23 is upstream’s current LTS; we have an agreement from upstream to support 1.19 for the lifetime of wheezy, and I guess they’d be willing to extend the same for 1.23 and jessie, but I’d not want to ask them to do that for 1.19.

Right. I wouldn't like to be stuck supporting 1.19 in Jessie. I don't think that is something we can agree to.

The delivery of the security updates from upstream to Debian (both stable and unstable) has been good so far, [...] so it’s been productive.

I'm glad to hear this.

If anyone’s got a rough overview of what changed between 1.19 and 1.23 for/from a packager’s PoV, thank you for pointing it out to me.

I'll try to put together a list this week.

I’m not formally the maintainer of src:mediawiki [but] I’m willing to step forward and do my share of the work.

Thank you!