This service collects only your twitter ID and your Mastodon Username and instance domain name. These are saved in the database to providing a link between your Twitter identity and your Mastodon identity. The list of users you follow on Twitter is accessed but never saved. No other data is accessed even if it is possible to do so based on the required permissions requested from Twitter to gain access to the list of users you follow. Twitodon has attempted to restrict the permissions requested to the minimum required to function.
When a you log into Twitodon and download the offered CSV file you are receiving a correlated map of Twitter user IDs of users you follow matched against those users' Mastodon username and instance name. If you do not want to be discoverable on Mastodon by users who follow you on twitter then do not use this service.
Twitodon does not utilise any analytics or advertising cookies, and does not sell any data nor make any data available to third parties.