The limit on past logs is size-based, not time-based. Specifically, it is 256 KB. This would pale in comparison to the size of your database backup, which I'm not even sure includes logs and is not a good indicator in any case (event history pruning and other maintenance tasks get performed every night in the early AM and may affect this size).
There were no documented changes to this in 2.1.9, and it's unlikely that there were any secret changes, either (they normally say something). So, if you're noticing that they aren't going as far back as they used to, the most likely explanation is that one device or app in your logs is particularly chatty (some "Dashboard tile aggregator"-stlye apps are like this, as are some community apps and drivers that don't follow Hubitat's convention of automatically disabling debug logging shortly after enabling it or adding the device).
That being said, it's possible they accidentally changed something here. I just checked a few of my hubs: my C-3 has about 96 KB of logs (if I copy and paste the text only and count characters/bytes; not sure what else Hubitat may store here) for about 1200 lines, about an hour of history. That hub is particularly chatty, so this doesn't seem too unreasonable, but it's still barely half as much as the others. MyC-5 ZHA/lighting hub has about 160 KB of text, totaling over 2000 lines and going back about 5 hors (this one just has info logging enabled and I'm careful about the devices and apps I use on it). A C-4 I only use for testing and development has about the same size and lines, except it's over 10 days on that thing. 
So, in my casual observing: there does seem to be some discrepancy here. I'm not sure if it's 2.1.9-related since I rarely check past logs back more than a few minutes (usually when I just noticed something odd or meant to have live logs open to intentionally test something but didn't), and it's possible it's always been this way. And again, it's not uniformly affecting everywhere I looked.