I went to Wi-Fi settings, "Edit" in top right, scroll to bottom "Managed" section, and was able to turn off "Auto-Join" for the "t-mobile" managed network just fine. I did this many months ago, I think because I was infuriated at the idea of auto-connecting to a Wi-Fi network I did not opt in to, but regardless, the checkbox has remained off through a few OS updates since (on 26.1 now with a T-Mo prepaid eSIM).

There's no "Managed" section showing up on my phone and the last time I set that network to not auto-join it still did. Lesson learned, I just turn off WiFi and Bluetooth before heading out to Home Depot.