Minecraft Classic was free for a long time, it's just that it was taken offline at one point.

(Note, Minecraft Classic ≠ "Minecraft Java" or whatever they're calling the primary pre-Microsoft PC version now)

There is Minecraft for raspberry pi edition which comes free with the standard raspbian desktop image. My daughter loves it. It ties in with the education stuff in that it has a python API. For the rest it is more limited compared to normal Minecraft.

My son tried to play Minecraft when he was 4, but he couldn't read yet so we gave him Blockcraft and later Roblox, both of which he can use without needing to read. That's because the interface is usable if you just click the defaults all the time, and he can clearly tell the difference between a blue button with text and a grey button with text.

Although funnily enough, his desire to play Minecraft motivated him to learn to read.

So it might be due to kids being exposed to and wanting to play these games earlier, which has perhaps caught Minecraft off guard resulting in a new generation that plays fast-following Minecraft clones instead of actual Minecraft.

Roblox doesn't work on Linux even in Wine.

My son (8 y.o.) went through that phase, but he realised that Roblox isn't nearly as good of a game, and he went back to Minecraft.