http://www.lmdb.tech/doc/group__mdb__env.html#ga492952277c48...
Ctrl+f through the doc for "MDB_FIXEDMAP" for more details.
So there is _some_ support, where you can expect to have the whole file at a fixed address. What is not handled is that your data may be moved within the file, and there's nothing you can do about it, short of keeping a long-lived read only transaction:
In practice, nobody has wanted support for this so it has remained unimplemented. Note that if your data items are large enough (more than 1/2 a page) they'll go into their own overflow pages, and then won't shift around at all. (But if you delete/modify a value, the new version will of course be in a new overflow page.)
Granted, this could be a case of "if you build it they will come" - before LMDB existed, nobody cared about mmap'd transactional DBs either. And personally, I still see good use cases for it.