Short answer: your own content always, public content usually, private accounts never. Long answer below.
Downloading your own Instagram content is unambiguously fine. You own the copyright to the photos and videos you publish, and nothing in Instagram's Terms of Use transfers that copyright to Meta. Instagram provides an in-app "Download your information" export tool precisely because they acknowledge you own your uploads. MediaFetcher gets you the same files without the 48-hour wait or the zip wrapper.
Downloading public content posted by other people for personal, non-commercial use falls under the same fair-use and fair-dealing doctrines that cover time-shifting, format-shifting, study, criticism, and research in most Western jurisdictions. The Electronic Frontier Foundation has consistently defended personal archiving as a lawful activity — the same principle that protected youtube-dl in 2020 protects downloading a Reel for offline viewing today.
Downloading from a private Instagram account is a different story. If an account has set itself private, Instagram requires a follower relationship to see any of its posts, Stories, or Reels. Meta does not serve that content to unauthenticated requests, which means no legitimate third-party tool can access it either. MediaFetcher does not attempt to bypass the login wall — anyone who claims they can is either lying, scraping briefly-public caches, or phishing for your Instagram password. Do not paste your Instagram credentials into any third-party site. Ever.
What about notifications? Instagram notifies Story authors when someone takes a screenshot of a disappearing photo or video sent inside a DM. It does notnotify anyone when a public post, public Reel, or public Story is downloaded via a third-party tool. MediaFetcher runs entirely outside Instagram's notification infrastructure — there is no account, no interaction, nothing logged against anyone's profile. The creator has no way to know a download occurred, because no signal is sent.
What is notlegal, anywhere, is commercially redistributing Instagram content you do not own. Downloading a photographer's Reel to study their lighting is fine. Re-uploading that same Reel to your own account for engagement is copyright infringement under Instagram's own Community Guidelines and under the laws of most jurisdictions. Saving a carousel from a brand for your personal mood-board is fine. Using that carousel in a paid ad campaign without permission is not. The line is always: personal use and study are fine, commercial reuse without a licence is not.
For the full background on Instagram as a platform and the history of its content rules, the Wikipedia article on Instagram is a good starting point.
This is general information, not legal advice. If you have a specific commercial use case, talk to an actual lawyer in your jurisdiction.