Slang meaning of "sheltered workshop"

Literally, a sheltered workshop is essentially a place where people with limited abilities are given employment. In extreme cases, they perform tasks which are essentially of no value, and it has been seen as a kind of babysitting for adults with disability.

When I have heard this word metaphorically it's referring to these attributes:

• Participants who would not "survive" outside that environment
• People are not meaningfully questioned, challenged etc.
• A general lack of relevance or usefulness of the work being done.
• Being paid despite all of the above.

So, calling academia a "sheltered workshop" is to imply that this is a safe place for people who have no useful contribution to make in industry, but can find employment nonetheless. It's highly disparaging both to disability advocates and to academia, of course.