It took a while, but you'll soon need a Microsoft account to play any of Mojang's games.
Minecraft developer Mojang has announced that a Microsoft account will be required to play its various games going forward. This includes Minecraft Java Edition, which has long existed on its own without being integrated into Microsoft's larger ecosystem.
In a blog post, Mojang explained that having a Microsoft account log-in increases security because of its support for two-factor authentication, not to mention connect all the Minecraft PC games under one account.
A single account also makes it easier for parents to control their children's gaming, and block chat and invites from unwanted people. Minecraft players who do not yet have a Microsoft account don't have to get one right now, but they will be required sometime in early 2021. New players just jumping into Java will be asked to create a Microsoft account, however.
This has no immediate effect on the features of Minecraft Java. That version will still exist separately from Bedrock, and the two player pools will not be mixed. In other words, this is more of an admin move rather than a gradual phasing out of Java - for now, at least.
The migration process will occur in batches, so look out for an email with the instructions soon. Once that process is complete, your Mojang account will no longer be used to play Minecraft Java, and Mojang is giving everyone who makes the move a free in-game cape.
For more details, there's an official FAQ you can read.