When/where was it announced that the last update was in March?

The latest update for the OP 6T was Dec2020 which is normal for OP's bi monthly update schedule. 

https://www.oneplus.com/global/support/softwareupgrade/details?code=PM1574156215016

I don't know, they are just not even close to what Samsung is promising. My 6t will be getting its last update in march and that's android 11 HALF A YEAR after the release, and they are generally very far behind with security updates except for the first 6 months to a year and with the 7 pro that period already passed.

You can search all over reddit and online to see that OnePlus is either very slow to roll out Android and security updates or the roll off of support of devices.  It's likely the OnePlus 7 Pro won't get Android 13 support, or you will be waiting a long time for it after it is rolled out to  other devices.

https://amp.reddit.com/r/oneplus/comments/b8kay1/security_updates_from_oneplus/

https://nerdschalk.com/oneplus-7-pro-5g-update/

Whatever you do, don't get the OnePlus, their software support is terrible.
 the S20 FE seems like the best deal out of the three

Paying new prices for a brand like OnePlus is a sucker move. Find a good quality 7T or 7P off of Swappa or eBay.

I got my OnePlus 7T pro for half the price and it's brand new. I found definitely recommend the S20FE 5G. It has a faster CPU than the Oneplus 7 pro and pixel.

I do like the warp charge of the OnePlus and yes I was decapitated on the fact that OnePlus has become slow on software updates. However so far I am loving my OnePlus 7T pro.

Get the 4a. I've been using mine for the last 4 months most of the day, and it doesn't feel cheap. Get the Spigen Thin Fit case and it's a very nice phone for cheap...

What phone to get?