RSS (Really Simple Syndication) is a web feed that allows users and applications to access updates to websites in a standardized, computer-readable format.
Mastodon offers RSS feeds as a built-in feature and it is immediately accessible both via a web browser and to RSS feed readers.
https://{instance domain name}/users/{your.username}.rss
This works for any user, not just your own, and doesn't require authentication.
For example:
https://{instance domain name}/tags/{hashtag}.rss
For example: