Afaics, it has been technically plausible to double-spend on the Core fork since the implementation of BIP65 in 2015, because OP_CLTV is a NOP2 on The Real Bitcoin fork.

Thus to double-spend on the Core fork, first broadcast a transaction containing OP_CLTV that can not be added to the Core fork until some distant time in the future and run a Real Bitcoin (TRB) full node to download the real blockchain and verify that your transaction was immediately added to the legacy blockchain. Then issue another transaction on the Core fork to double-spend.

However there is a risk if the transaction on TRB is later orphaned and your Core transaction is replayed on TRB chain then you can lose your BTC. So the safest would be either to exchange your TRB tokens for fiat or another token, or mix into your Core transaction some UTXO that derives from a SegWit transaction, so that it can’t be replayed on TRB because TRB doesn’t implement SegWit. Do not issue a SegWit transaction (as opposed to mixing in a UTXO that derives from a SegWit transaction) because your BTC could be stoken on TRB if you do. The problem with incorporating SegWit is that you have to do it before buyers are smart enough to realize that they do not receive TRB when the transaction has any SegWit history. And buyers may be wary of SegWit transactions any way because it is new.

However, the problem is that it is probable that no miner is still issuing blocks that enforce the protocol rules of The Real Bitcoin. The changes in the protocol have not yet incentivized a fork, thus although The Real Bitcoin does not reject OP_CLTV transactions, it also probably has no way to get them into a block earlier than the Core fork will.

There is no sense in mining your own Real Bitcoin block, because later when some significant hashrate decides to also do so (in order to spend all historic SegWit and derivative transactions to themselves), it is not like they will build on top of your block.

So the above technique can be used (to make double-spending more deterministic, i.e. you will not need to try over and over again) as soon as a significant amount of hashrate has started enforcing The Real Bitcoin protocol, which means it will not delay OP_CLTV transactions since they are a NOP2 in the legacy Satoshi Bitcoin.

I do believe The Real Bitcoin will ultimately be the fork that survives and prospers because SegWit writes (as explained more in my blog) a free futures option for legacy Satoshi Bitcoins (aka TRB) on all user funds that pass through any SegWit transactions such as Lightning Networks. Whales will choose (rally around the only Schelling point) to sell the fake Bitcoin that is not secure for many reasons, and buy The Real Bitcoin.