If you want to protect yourself from the rampant toxicity often found on Twitter, one way to do this is by making your account private through a feature called "Protect my Tweets." It does things like only showing your tweets to approved followers, makes people send a request to follow you, and more. Unfortunately, if you use the Twitter Android app and had this turned on, there's a chance it may have been disabled without you knowing it.
On January 17, 2019, Twitter shared the following:
We've become aware of an issue in Twitter for Android that disabled the "Protect your Tweets" setting if certain account changes were made. You may have been impacted by this issue if you had protected Tweets turned on in your settings, used Twitter for Android, and made certain changes to account settings such as changing the email address associated with your account between November 3, 2014, and January 14, 2019. People on iOS or the web were not impacted.
Twitter says it fixed the issue on January 14, but even so, that's well over 4 years of this being around and affecting users.
People that were impacted by this are being contacted by Twitter and having Protect your Tweets turned back on for them, and if "other important information becomes available", Twitter says it'll let us know.
Should you have any questions about the matter, Twitter encourages you to contact its Data Protection Officer, Damien Kieran, through this form.