Sure. I did a quick search for a mod that might already do what you want, but, if there is one, then enabling these popups would perhaps be too minor a change to be documented. Anyway, I didn't find one, but it turns out that EmperorFool already posted the code snippet that needs to be changed
here 10 years ago.
The first line needs to be changed to
if (isHuman() && getCurrentEra() != GC.getGameINLINE().getStartEra())
Guide for compiling the DLL
If you go by that, then be sure to install Visual Studio for C++ (not for Basic) – that may be obvious, but I've recently seen two people make that mistake.
You can find
CvPlayer.cpp in the
CvGameCoreDLL folder in the BtS install directory. However, if you download Leoreth's Git repository with the Visual Studio project files, then that download will also include a copy of
CvGameCoreDLL, so you might as well work from that copy. The game does not load any of the files in
CvGameCoreDLL, it only loads the compiled
CvGameCoreDLL.dll from
Assets.
I wrote that you "may want to" create a mod, but, come to think of it, I'm not sure if multiplayer will still work if one player changes the original files and the others don't. (Since these popups don't change the game state, getting out of sync shouldn't be a problem, but the game might not allow players to connect.) If you create a mod (which only means creating some folders), then all players will have to install and load that mod.