Do there exist robust structures in Conway's Game of Life? For instance, has anyone constructed a spaceship with a shield that absorbs all small oscillators and gliders it collides with?
See "eaters" category.
They can absorbe gliders and spaceships, but I'm not an expert and I don't know if there are "walls" capable of absorbing everything. Perhaps it is a nice subject to investigate! (see the references at the bottom of Wikipedia pages for modern approaches using constraint solvers).
From the lexicon:
:eater Any still life that has the ability to interact with certain patterns without suffering any permanent damage. (If it doesn't suffer even temporary damage then it may be referred to as a rock.) The eater1 is a very common eater, and the term "eater" is often used specifically for this object. Other eaters include eater2, eater3, eater4 and even the humble block. (In fact the block was the first known eater, being found capable of eating beehives from a queen bee.) Another useful eater is shown below, feasting on a glider.