Hrm. 2.0.4 is well over 2 years old and SDL has since had numerous bug fixes in addition to the couple of new features. Limiting resizability changes to window creation time isn't a huge burden but let me audit the other changes that were made since 2.0.4 to see if we'll be missing out on anything major. Thanks for the info.
New Ubuntu LTS(18.04) should come out this week probably shipped with SDL2 version 2.0.8 on the board, and since it's LTS, supporting older LTSes may not be that necessary anymore.
Debian stable has 2.0.5 though. Enough for resizing window, but not for vulkan goodies.