Magic: The Gathering has been an ever-shifting game for the past 26 years. To put that into perspective, I was only a year old when the game came out. I don't feel any younger saying this, but as a truth, it needs to be said. Regardless of the age of the game (or this author), with a game that is in a constant state of flux with new cards being released more than every fiscal quarter, there are bound to be heavy shifts to the rules of this game. As Magic goes into releasing its 82nd expansion set, Throne of Eldraine, Wizards of the Coast has graciously provided its player base with a reasonably-sized list of comprehensive and "Oracle" rules changes.
The Comprehensive Rules updates pertain primarily with new set releases and the new set of mechanics emerging from therein. Usually, this is solely the case unless errata is ever issued with regards to certain rules interactions. While I am not going to spell out the extensive list of updates to the Comprehensive Rules, I will mention that most of it deals with updates from Commander 2019 and Throne of Eldraine. The list of Comprehensive Rules changes can be found here.
As a bit of a breath of fresh air, the Oracle rules changes are mostly pertinent to making errata changes to cards themselves. This has historically mostly been full of nonfunctional changes but occasionally functional errata will be put into place and change how a card or cards act and interact. For example, The Grand Calcutron is now a card with, in addition to its original text, Oracle text that reads "The Grand Calcutron can be your commander." – which even then is not something Wizards of the Coast does to black-bordered cards (they have since apologized to the Nephilim cycle).
The Oracle rules changes can be found here.
The changes made for this game are healthy and crucial for the longevity of Magic: The Gathering as a whole (including Magic: The Gathering: Arena, by the way!). Without change, the game cannot adapt, so it's good that the design and development teams acknowledge the need for changes to be made. Let's hope that Magic continues to flourish with these small yet numerous changes!