Upon loading the old launcher, users will be greeted with an explanation as to why their method of playing the game will soon no longer work.
In the second quarter of this year, Mojang will be discontinuing this old launcher, forcing players to use the new, updated one. In addition, they will also drop support for Windows XP and Vista, meaning that players on these operating systems will no longer be able to play newer versions of Minecraft.
The reasoning for this, says Mojang, is
the approaching transition from Mojang accounts to Microsoft accounts set to occur this year. This account migration brings Java Edition accounts in line with accounts on the Bedrock editions which have used Microsoft accounts for a while now.
If you are still using the old launcher, you will have to download the new launcher if you want to be sure you will be able to log in and play the game.