Assuming a reasonable amount of searching has already been done, is it OK to ask for maintenance/technical manuals for old hardware here?
A qualified 'yes'.
Since at least some of the forum members actually do retrocomputing, if anyone's going to have Ye Anciente Servysse Scrolles, they might.
The fantasy that StackExchange is producing an encyclopedia of answers that will remain valid for ever means that anyone foolish enough to volunteer having such documentation available will become the permanent source for same.
Finding a manual is a feat in itself - even with several 'reasonable' sized shelves of manuals I'm in constant search for manuals and data sheets. So yes, some information is really hard to come by (*1) without knowing the right chain of search terms, or in which covering books they may be hidden.
I do not see the long term gain as small, like Another-Dave mentioned. All it does is restricting usefulness to exactly what is asked: Finding that book. No real difference from any other question.
Toby Speight's prerequisite of details about for what the book is searched does make a good basic request for similar questions. It may be essential to find the right answer as often the desired book doesn't exist or at least not on its own.
Let them keep coming ...
... and take such question with considerable caution and weed out the obvious lazy ones - like it should be done anyway (*2) - maybe being a bit more tight than usual.
*1 - Just for the fun of it, try to come up with a data sheet for TI's MSC0430.
*2 - If you haven't already, check the tooltip for downvoting questions :))