So Mat Block is still in Sword and Shield, exclusive to Throh... as a transfer only move... why. It used to be Greninja's signature move but was added to Throh's moveset in gen 7. In Sword and Shield, Throh can no longer learn it by level up, and it's not even useful for Throh to have! Mat Block protects the user and the partner from any attacking move, but does not have priority. It ended up having some sparse use in VGC on Greninja because, you know, Greninja has this little thing called a "speed stat of 122." Usually you would be better off with a pokemon with Fake Out, but it had a niche.
Now lets turn to Throh, Throh has a speed stat of, uhh, 45???? yeah 45. So could use it under trick room or if you speed swap with another pokemon, but its not what anyone would call viable. There is a lot of set up needed for Throh to pull off a Mat Block, and you are probably better of using Fake Out or Greninja if you are playing a Gen 6/7 game. Well now it's the only Mat Block user in Gen 8, so why not give it a go! Oh, Throh cannot learn Mat Block by level up, it is only a transfer move, so Throh cannot use it in the official Gen 8 formats.
Why?
I'm not trying to hate on Throh so much, but why keep Mat Block when its: 1) Transfer only, 2) a former signature move for a pokemon not in the game, 3) not really useful on the pokemon in question, and 4) not even usable in the official competitive format of the game. Maybe it is just a forgotten relic from development, since we know that Greninja was used in the leaked development versions, but it seems odd to keep this move over everything else.