Mar 24, 2025

class CTransaction
{
public:
...
    int nLockTime;
...    
    bool IsFinal() const
    {
        if (nLockTime == 0 || nLockTime < nBestHeight)
            return true;
        foreach(const CTxIn& txin, vin)
            if (!txin.IsFinal())
                return false;
        return true;
    }
...
};

class CTxIn
{
public:
...
    bool IsFinal() const
    {
        return (nSequence == UINT_MAX);
    }
...
};

class CTransaction
{
public:
...
    unsigned int nLockTime;
...
    bool IsFinal(int64 nBlockTime=0) const
    {
        // Time based nLockTime implemented in 0.1.6,
        // do not use time based until most 0.1.5 nodes have upgraded.
        if (nBlockTime == 0)
            nBlockTime = GetAdjustedTime();
        if (nLockTime == 0)
            return true;
        if (nLockTime < (nLockTime < 500000000 ? nBestHeight : nBlockTime))
            return true;
        foreach(const CTxIn& txin, vin)
            if (!txin.IsFinal())
                return false;
        return true;
    }
...
};

// To help v0.1.5 clients who would see it as negative number. please delete this later.
if (nLockTime > INT_MAX)
    return error("AcceptTransaction() : not accepting nLockTime beyond 2038");

So absolute block locktime was always active, it was forked in v0.2.0 to include time

and that was immediately applied to all blocks.