If you are interested in reading the forums in a similar way to reading the latest entries in a newsfeed ("Just In", etc), try these links.

Also makes it easier to do a search of the last few days.

For Sqlite & Fossil, I get the latest 100:

https://sqlite.org/forum/timeline?ss=v&y=f&n=100&vfx&nsm
https://fossil-scm.org/forum/timeline?ss=v&y=f&n=100&vfx&nsm

The Pikchr forum isn't changed often, I get the latest 50:

https://pikchr.org/home/timeline?ss=v&y=f&n=50&vfx&nsm

Is there a query that will just show topics (one line per topic) that have had postings in the past 24 hours?

The days=N parameter is what you're looking for, but there's not a way to reduce the output to just the topic strings.

Example: /timeline?days=1&y=f

(Note that "N days" really means "N times 24 hours," so days=1 may span 2 days.)

Edit: adding &ss=c to the URL flags will reduce the output further but won't eliminate the timestamps.