Soft Fork – Bitcoin Client Backward Compatibility

I think the linked answer mostly answers your question, but if not, I’ll address the specific example you raised.

I wonder if if I run version 0.1.0 of Bitcoin today, it will sync and work normally.

No, it’s not because of changed block/transaction validity rules, but because the P2P protocol has changed (early packets have checksums now since 2012 and are not supported by nodes before 0.2.10). I believe it is possible to write a simple proxy to solve this problem (add the checksum in one direction and remove it in the other). In this case, modern nodes can communicate with very old Bitcoin code.

Another problem is that peer discovery has completely changed. 0.1.0 Nodes do not look up the IP addresses of peers to talk to. This issue can be resolved by connecting manually.

I understand you were only using P2PK at the time, will 26.0 reject this?

This is not correct. The “1…” address format corresponding to the P2PKH output was already supported in 0.1.0. Moreover, the Bitcoin Script system has existed as a consensus rule since 0.1.0. All modern address formats are valid scripts according to the first release. Also, in the other direction, when using 0.1.0, the addresses and outputs of the built-in wallets (P2PKH and P2PK) are still perfectly valid and standard by the latest software, allowing you to receive payments from the latest software.

If your question is whether 0.1.0 can do it Do business with It’s a little trickier (as is creating a transaction that’s accepted by the network). A number of agreements and policy rules have changed. I don’t think any of these really matter, except for the BIP62 low-r policy rule. Because of this, only a small percentage (25% or less depending on circumstances) of transactions created in 0.1.0 are allowed by the latest rules.

And there have been many changes, such as OP_RETURN being changed from 80 bytes to 40 bytes, so what happens to the soft fork?

This was just a policy change and did not affect the consensus rules, so it had nothing to do with the soft fork.

there have There have been soft forks (earlier hard forks) since the 0.1.0 release, but none that really affect the current chain. So, to be clear, I believe the current Bitcoin blockchain will be fully valid as of 0.1.0 (but see the linked answer – it will be very slow and require configuration changes to prevent it from falling over).