Grow and Shrink effect is defined in /usr/lib/libreoffice/share/config/soffice.cfg/simpress/effects.xml. However, you cannot find it in the custom animation menu.

To add it, do the following:

Open the file /usr/lib/libreoffice/share/registry/impress.xcd and search for the word "blink"
You will find two occurrences of the "ooo-emphasis-blink" effect. The first one is the definition and the second occurrence is where we tell LibreOffice Impress to show it in the custom animation menu.

Now search for "ooo-emphasis-grow-and-shrink" ans you will see that it's only found once. It is actually only defined.
Now go to the second occurrence where you found ooo-emphasis-blink and add ooo-emphasis-grow-and-shrink there as well.
Restart LibreOffice and Grow and Shrink effect should now appear.