Where did Data Saver go?

tl;dr Data Saver setting vanished - factory reset makes it come back.

I have a Samsung S21 with the most recent updates using the Visible (Verizon) network. In the middle of the month, the Mobile Hotspot stopped working. It declared that it couldn't turn on until the Data Saver setting is turned off. 

Except the entire Data Saver setting is gone. Not just grey'ed out, entirely missing. 

Every search returns results on how to get to the Data Saver setting in order to turn it off. Except the Data Saver setting is NOT THERE. 

There are no updates available, reboots made no difference, and I was out of ideas. Fortunately, a factory reset corrected the issue. 

I'd love to know why it disappeared, if only to prevent it from happening again. 

Search terms (from last to first): 

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S21 5g visible Verizon data saver 

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s21 data saver "missing" 

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what the fuck is data saver Android 

Postscript: 

Why does my phone have ATT apps on it? It didn't come with ATT apps. I have only used Visible as my provider. Why does ATT think it's okay to force install spam games on it's customers? Humbug.