Sega announced via their livestream that Sonic Mania for the PC will be delayed until August 29th to allow for more time for testing and better performance. If you pre-ordered the PC version, you'll be treated with a free version of Sonic 1 (not the updated Christian Whitehead version) for your troubles. If you pre-ordered the Collector's Edition, you'll still get the box and the download code will still work but will initially release Sonic 1 until August 29.
Edited to add their official statement
In Sonic Mania, quality and attention to detail are some of the most important things for our team. We accept nothing but the best for Sonic Mania, and that’s why we want to spend a little more time to polish the PC version of the game’s release, which we plan to complete by August 29th. All other platforms (PS4, XB1, Switch) will still release on August 15th as planned.
We know you're all super excited to play the game on PC, and as a thank-you to everyone who has purchased the PC Collector's Edition, or pre-purchased the game on Steam, we will be unlocking Sonic 1 for free on your Steam accounts on August 15th. If you already own Sonic 1 on Steam, you’ll be given a giftable version that you can trade or send to a friend.
On behalf of everyone involved with Sonic Mania - thank you very much for your patience. We can’t wait for you to get your hands on Sonic Mania soon, and we hope you enjoy playing it as much as our teams enjoyed making it!