Sanctions sometimes ban providing services to people in countries.
Apparently Minecraft requires you to login (a service) to use the game. If the game was completely offline this wouldn't be a problem.
Barely a week goes by without some variant of "Iranian Hackers" in the tech/security press - maybe, inshallah, they could release a version that lets you play on Iran-hosted servers without needing to connect to the US for a login?
No, they don't need a 'new' client for that. That already exists. Since minecraft is basically open source, it's not really a secret that how to make a client that simply don't require login. (Surely you don't want to say it loud or legal team of MS is going to chase you hard, but lots of client just slip that function in silently)
Besides that.
The minecraft login is basically a license check allow you to join 'legal purchase required' server. And whether you want to enable the enforcement is up to server owner.
Open source means the developers release the source code willingly and encourage other people to use it. Minecraft is not now and never has been open source.
They do release the source mapping of the java version alongside the binary. Which can be used to decompile the whole game back to compilable java file. (And you can fiddle it around starting from there). It's source available if you do own the game. So I say 'basically'.
The bedrock version on the other hand is completely proprietary. With very little extension api exposed. So there is very few mods of them.
That's decompiled source and while, for Java at least, it can be pretty usable, it's actually going to differ in many ways from the original source due to optimization and obfuscation. I'm fairly sure neither Mojang nor Microsoft were particularly pleased about this, indeed their use of obfuscation implies they want to make your life more difficult doing it.
I think your point is that the Java version at least is easy to modify, but that's because it's in Java, not because Microsoft wants you to. "Open source" and even "source available" are terms of art which don't apply here. "Very hackable" seems more appropriate.
Before this exists, there is a community project to de-obfuscate naming back to readable human names. And it usually took about a month after new minecraft version released. But this is no longer needed because mojang now release the actual mapping.
So microsoft(mojang) do want to make modders live easier out of own willing. Not just because it is java. Or they don't really need to publish that at all.
This reply is probably a little late, but that check is just a line in the config file. You change something like verify-purchase=true to verify-purchase=false, and it's done.