Are links by link shorteners like goo.gl, bitly or others  valid as long as the underlying documents?

No, until the company is bankrupt or the end of mankind.

If the underlying document is vanished/moved the shortened url will end in a 404.

 

One that must have been valid in 2014.

https://goo.gl/OmwwgJ

AND

is dead, 404.

So the goo.gl works but the document is gone.

Must have been something about GSM repeaters...

Used in a Thaivisa post by me in February 2014.