A soft fork means that old code will accept the new rules
nope, that's wrong
Yeah I agree with Carlton here. The segwit soft fork meant that legacy blocks are still accepted after the fork but this doesn't work the other way... From what I can tell the signed part of segwit transactions just get ignored by full nodes before v13 and they just take the 150 byte transactions (as an example from the standard size) because the longest chain is running with them (and there isn't a reasonable rival).
I sorry to say that, but this is still wrong. See here for what a soft fork is:
https://en.bitcoin.it/wiki/SoftforkI.e. it means that only additional restrictions are added (in the case of segwit, that the witness must be valid in addition to the old-style signature). This means that
old nodes accept all new blocks, but it also means that
transactions that would be accepted by old nodes may be invalid according to the new rules.
Now it could be that in the particular case of segwit mining, you are fine running old code, as long as noone maliciously sends you transactions that are valid according to old rules but invalid according to new ones. But that would then be just by chance and not a general property of soft forks. Also, at least according to my understanding (I'm not a miner and haven't looked at the details recently), aren't segwit blocks supposed to have a new commitment in the coinbase, which old mining software wouldn't create?