That means, in short, all bitcoin client releases were not backward compatible in nature. Does not it lead to the fact that Bitcoin has hard forked before?
No, Bitcoin has not hard forked before, and all releases are actually backwards compatible since all protocol changes have been soft forks. As 2112 said earlier, you can actually get past the BDB lock issue with some special configuration. So in theory, you could sync a 0.1.0 client fully up to date with some special configuration.
Is not it saying that you can validate all Bitcoin blocks with
with some special configuration of Bitcoin Cash or Bitcoin SV client? So, those chains are not hard forked either!
Same can be stated about Ethereum. Ethereum Classic chain can be validated with Ethereum client
with some special configuration. So, Ethereum has never forked!
Are not we getting irrational here?