Until very recently I could password protect a document by selecting Get Info and checking the 'Locked" box but this function seems to have disappeared.
Are there any other users experiencing this?
That has never password protected a document. All that checkbox has ever been for, throughout 27 years of Mac history, is for preventing accidental changes or deletion.
If you want to protect a document, perhaps the app itself provides that function. Microsoft Office apps, for example, can encrypt their files with a password. Otherwise, you'll need to get an encryption utility or create a password protected disk image using Disk Utility: