Really the only answer this thread needs. Companies can be bought and sold and the information they have can change hands and change retention and privacy policies. There's no trustable verification method for knowing, for absolute certain, that none of your data has been retained at your request. There simply is no reasonable way to trust these services.
Makes me wonder if there could be a business model, type (maybe nonprofit?), or license that user data would be destroyed in case of a change of ownership. Or maybe even a time-limit on data, like how messengers have self-destruct options.