There is no official confirmation of this anywhere that I can find, but according to Jeremie Boga's representative (who apparently is also his brother?), the 19-year-old highly promising prospect signed a new contract with Chelsea before heading out on loan for the season. This would certainly fall in line with our recent habit of extending players before sending them out to their temporary homes.
That all comes from a report at Foot Mercato, which features quotes from the aforementioned brother and representative Daniel Boga. Quotes from family member should always be taken with a grain of salt (right, Mr. & Mrs. Edinson Cavani?), but I don't see a good reason for brother Daniel to make such things up, so presumably he's truly in the know.
You'll note that a contract expiring in 2021 means a six-year contract, which used to be against regulations in England. But Kevin De Bruyne just signed a six-year deal with Manchester City as well, so perhaps this restriction was lifted recently? FIFA do allow contracts of this length (and teams in Spain, for example, have been giving them out for some time now), but the governing bodies in England used to limit the maximum length to just five years. Hmmm...
In any case, Daniel Boga also reveals a few additional details of how Jeremie ended up at Rennes, when it looked like he would go to either Rayo Vallecano (deal fell through at the last minute, apparently) or Olympique Marseille (couldn't meet playing time guarantees). Other French and Spanish teams were interested as well it would seem, but Rennes got their wish in the end. And they got a (€40m?) buyout clause as well, which remains a really weird facet of this whole thing, especially in light of the claimed contract extension at Chelsea.
Boga's first chance to make his Ligue 1 debut for Rennes will be this coming weekend, after the international break.