Samsung Galaxy S8 quarterly updates

It's the only way your going to get them.  Att has to push them.  They updated the S7 when it was quarterly so I would say you will see the updates.

But, did AT&T ever push out the March 2020 patch for the S7?   The support site says they are still on the Dec 2019 patch.

The S8 should be getting the quarterly security patches, but you can tell AT&T starts losing interest in keeping their older phones updated after a while.

According to this, there's a security update with the June 1, 2020 security patch, but my phone says it's up to date, with the March 1, 2020 security patch.

https://www.att.com/device-support/article/wireless/KM1203359/Samsung/SM-G955U

I did get the June 1 update on my S8 a week ago. About the same time my S9 got the July 1 update with One UI 2.1 (which broke the wallpaper scrolling feature). 

I contacted AT&T support via chat yesterday. August 2. The agent said they would push the update to my phone in 2 - 3 hours. Six hours later, I contacted them again and they again said I would get the update soon and that a supervisor would contact me. Well, it's been over 24 hours since I first reached out to them and no update.

I'm afraid you're stuck. Far too often, users here said AT&T CS says they will push an update, but nothing ever happens.

My belief is that once you miss an update, you're forever toast. No matter what they tell you, AT&T lost your phone in their system and there's no fixing it. 

What's more frustrating is that my family has four S8 phones and none have gotten the update.

After doing a factory reset, the phone still wouldn't update to the latest security patch.

Let's see what happens with the September 2020 update that Samsung just released. Hopefully AT&T releases it in a timely manner.

https://9to5google.com/2020/09/21/september-patch-now-rolling-out-for-galaxy-s8-series/