Only if you let them
Your keyholder can pay for your Plus only if you let them. On Sharing, under What they may do, switch on Offer my keyholder the chance to pay for Plus. It is off unless you switch it on, and readers are never offered it.
What it changes
Nothing about what they see. They still see only what you switched on for them, and you can change that, pause them or stop sharing at any time. Settings shows your Plus as coming from your keyholder.
Who can change it
It is theirs to change or cancel, on checkout.clackis.com. If it ends, everything you made is kept, as with any Plus ending. You can delete your journal without waiting for a gift to end.