I should imagine ransoms are payments are tracked. Tracking who gets paid for the ransom is surest way to end them. And it only takes one mistake to stain all the transactions.

Deniability is hardly the point - all dodgy transactions are deniable, it's just whether people care to pretend to believe you - the UK has anti-money laundering laws that basically say if you cannot reasonably prove your money is it dodgy we will take it. https://news.sky.com/story/zamira-hajiyeva-supreme-court-rej... - the criteria here is basically "do we believe do you" - it's really nebulous.

All it takes is a mixer with a large enough anonymity set.

The larger the set the greater likelihood of a coin designed to track joining ?

I'm not sure what your question is. In brief a mixer could works like the following:

Multiple people put money into a pot, in return they get an voucher that anyone can redeem.

At some later point in time someone redeems some money from the pot.

The anonymity set for this mixer is the number of people who put money into the pot. The more people that use this, the more anonymous.

This is the best explanation in the whole thread.