So yes, you don’t really lose anything by doing this. Is it easy to detect? If you have enough data, you should probably be able to do it. i.e. look at all the validators and if you see one that consistently doesn’t include any delegation transactions, you can see it.
But my bigger point is that it does me no harm. So maybe when you’re the proposer, you leave me out of the block, but the next validator includes me. I don’t care. Even if 90% of the validators do this, I don’t care. It just means it will take 10 blocks on average to get in so I lose 1 minute worth of earnings.
It starts to sting if I just can’t get in at all. But for that validators don’t just need to exclude my transactions from a block, 1/3 of validators have to refuse to sign (and thus finalize) any block that excludes my transaction.