Why did my download speed drop from 250 to 150 on June 21, 2022

You speed test through the router?  For accurate tests, you should direct connect to the modem, unless, of course, you have an all-in-one device for a modem & router.

You've got pretty consistent historical results.  If you have an all-in-one device, you must disconnect all other devices during tests, huh?

So...if you subscribe to any other plan bigger than 150 Mbps, such a 500 Mbps, you will always be limited to 150 Mbps with Traffic Statistics enabled on this router?

Apparently so. FYI TP-Link 1200 AC -> Arris TM 3402A. Whoda Thunk? Plan is now 250/10 with automatic speed increases from Cox on the premier plan. Been that way since 1999 when it was 50. BTW, I never noticed the speed drop in my daily use until I did a speed test just for something to do.