This bug can be achieved by gliding with your Elytra and teleporting to another location whilst gliding.  After some testing it seems that the easiest way to achieve this bug is to glide through the exit end portal, and once teleported to spawn you will stay in your gliding state, even if you remove your Elytra.  It seems as if you need to be teleported on a full block however, or this bug will not work.  This may be caused by the small spike of lag that comes when loading into new chunks.  An example is provided.

     It seems this bug has been around since 2015 and is still not resolved.  It's easy to replicate, but seems to have very specific factors that need to be fulfilled before it can be replicated, or may be somewhat of a random occurence.  However out of 10 attempts, I got to replicate it 8 out of those 10 attempts.

(NOTE: I am on a realm, with somewhat buggy chunk loading.)