A developmental football league for veteran free agent players has been created, but it has no official connection to the NFL as of now.
Per Pro Football Talk, the spring league has "no connection to the NFL", and is being created by the person who ran the Fall Experimental Football League. Furthermore, the league sent a memo via PFT to all NFL teams regarding the league providing guidance on sharing video footage with Spring League scouts and signing players to Spring League contracts.
The Spring League, which begins in April 2017, also affirms it has no affiliation with the NFL, although it hopes to establish an "official" relationship with it.