This site is centered on the Hedgehog Platformer extension inside the HedGD framework.
HedGD is a Sonic The Hedgehog game framework for GDevelop, the no-code 2D (and now 3D) game engine made by Florian "4ian" Rival.
It includes a modern scene, this one has Sonic Rush moves like the jump, roll, spindash, stomp and boost.
There's also a 3D modern scene that is inspired on Sonic Robo Blast 2 and Sonic Unleashed, with the moves of previously mentioned Sonic Unleashed like jump, boost and drift.
This one uses GDevelop's 3D extension "WithThreeJS" made by PANDAKO. You can learn more about that extension here: www.youtube.com/watch?v=f8xeEB59GLA&t=79s&ab_channel=EnjoyMakingGames
There's also another build with a classic genesis-style of HedGD framework developed by UnknownUney which is currently in development and is expected to be equiparable to the mega drive's Sonic The Hedgehog games.
Keep in mind that this is not an update for the extension: Sonic Develop. As this framework uses a completely new extension made from 0, but this extension could be considered as a "Remake" of Sonic Develop.
Well yes, the framework is free to use and/or modify in any project as long as the credits are gathered.
Keep in mind that you can contribute to the framework by sharing improvements/additions of it.
Just remember that you can't monetize with Sonic Fangames if you use the Sonic IP, you better make an original game for it.
Contributions to the framework can be accepted by using the framework and/or promoting it, you can also contribute by coding functions on it and by creating templates & games with it.