I can see larger thumbnails by default

What does that mean? T65440#1467035 ? T51357 ?

Which specific "new thumbnail border changes" (link) is this about?

I think this was about changing the border style around thumbnails, but I can't remember the details.

@Gilles: So this task is not actionable due to its vague description and should be closed? Or what does that mean?

I CCed the design team in my above comment, as they might remember better than I do. @Pginer-WMF?

I was not directly involved on this (@Vibhabamba or @violetto may have more retails) but I recall there were some experiments to do the following:

• Increase the default size of images. Currently, images by default are presented like miniatures that you need to click on them to properly view basic details. Articles can become more rich visually by increasing the default size of images without . With bigger screens being available, it seems possible to increase the default image size while still leaving enough room for the article text. I don't recall which size was proposed by default, but I think it was a moderate increase not a radical change for edge-to-edge images in the style of The New York Times experiments.
• Simplify the borders around images. In order to reduce clutter around the image which may distract from it, it was proposed to remove the boxes. This style seems to be adopted in the mobile web (example) but is not supported on desktop. I recall some discussions about the removal of borders (e.g., problems when the image has relevant white space such as the flag of Japan), but I don't know if any further action was planned or taken.

The connection with Media Viewer mentioned in the ticket is basically about timing. Some of the experiments above were considered in a point in time where many changes were happening in response to community feedback, and experimentation on anything affecting how images was displayed could create confusion so the Multimedia team asked for experiments affecting images from other teams to be postponed for a while.