Как вывести список расширений X-сервера в Ubuntu 16.04?

Вы можете использовать параметр -queryOptions в xdpyinfo:

SYNOPSIS
       xdpyinfo  [-display  displayname]  [-queryExtensions]  [-ext extension-
       name] [-version]

DESCRIPTION
       Xdpyinfo is a utility for displaying information about an X server.  It
       is  used to examine the capabilities of a server, the predefined values
       for various parameters used in communicating between  clients  and  the
       server,  and the different types of screens and visuals that are avail‐
       able.

       By default, numeric information (opcode, base event, base error)  about
       protocol extensions is not displayed.  This information can be obtained
       with the -queryExtensions option.  Use of this option on  servers  that
       dynamically  load  extensions will likely cause all possible extensions
       to be loaded, which can be slow  and  can  consume  significant  server
       resources.

Пример:

$ xdpyinfo -display :0 -queryExtensions | awk '/^number of extensions:/,/^default screen number/'
number of extensions:    29
    BIG-REQUESTS  (opcode: 133)
    Composite  (opcode: 142)
    DAMAGE  (opcode: 143, base event: 91, base error: 151)
    DOUBLE-BUFFER  (opcode: 145, base error: 152)
    DPMS  (opcode: 147)
    DRI2  (opcode: 154, base event: 102)
    DRI3  (opcode: 149)
    GLX  (opcode: 155, base event: 104, base error: 169)
    Generic Event Extension  (opcode: 128)
    MIT-SCREEN-SAVER  (opcode: 144, base event: 92)
    MIT-SHM  (opcode: 130, base event: 65, base error: 128)
    Present  (opcode: 148)
    RANDR  (opcode: 140, base event: 89, base error: 147)
    RECORD  (opcode: 146, base error: 153)
    RENDER  (opcode: 139, base error: 142)
    SECURITY  (opcode: 137, base event: 86, base error: 138)
    SGI-GLX  (opcode: 155, base event: 104, base error: 169)
    SHAPE  (opcode: 129, base event: 64)
    SYNC  (opcode: 134, base event: 83, base error: 134)
    X-Resource  (opcode: 150)
    XC-MISC  (opcode: 136)
    XFIXES  (opcode: 138, base event: 87, base error: 140)
    XFree86-DGA  (opcode: 153, base event: 95, base error: 164)
    XFree86-VidModeExtension  (opcode: 152, base error: 157)
    XINERAMA  (opcode: 141)
    XInputExtension  (opcode: 131, base event: 66, base error: 129)
    XKEYBOARD  (opcode: 135, base event: 85, base error: 137)
    XTEST  (opcode: 132)
    XVideo  (opcode: 151, base event: 93, base error: 154)
default screen number:    0