Apologies for the delay, was working/troubleshooting same issue and was away from my emails. :(
Regards...
Received the database with huge pg_multixact directory of size 21G and there are ~82,000 files in "pg_multixact/members" and 202 files in "pg_multixact/offsets" directory.
Did run "vacuum full" on this database and it was successful. However now am not sure about pg_multixact directory. truncating this directory except 0000 file results into database start up issues, of course this is not correct way of truncating. FATAL: could not access status of transaction 13224692
Stumped ! Please provide some comments on how to truncate pg_multixact files and if there is any impact because of these files on database performance.