Where do I find a list of all X keysyms these days?

Probably the best up-to-date values for key symbol definitions is to look at the source-code.

Basically its just a list of keysym names with their associated codes.

sudo apt-get install x11proto-core-dev

Two key keyfiles in /usr/include/X11:

The main definition file:

/usr/include/X11/keysymdef.h

Vendor specific (i.e. Debian/Ubuntu):

/usr/include/X11/XF86keysym.h

There are a number of other header files in the same folder you can also examine:

$ grep -l '#define.*XK_' /usr/include/X11/*.h

Example definition from /usr/include/X11/keysymdef.h:

#define XK_BackSpace                     0xff08  /* Back space, back char */
#define XK_Tab                           0xff09
#define XK_Linefeed                      0xff0a  /* Linefeed, LF */
#define XK_Clear                         0xff0b
#define XK_Return                        0xff0d  /* Return, enter */
#define XK_Pause                         0xff13  /* Pause, hold */
#define XK_Scroll_Lock                   0xff14
#define XK_Sys_Req                       0xff15
#define XK_Escape                        0xff1b
#define XK_Delete                        0xffff  /* Delete, rubout */