Not to resurrect the sad old horse that’s been dead and decomposing for years, and then beat it some more, but the test you propose has been done:
Is ESP8266 5 V Tolerant? This Curve Tracer Says Yes! | Hackaday
There’s no need to pile on here: the comments on that article point out all the different reasons why those test results shouldn’t be trusted. (I had a Nano and ESP8266 talking together for about 18 months, without level shifters, with no issues; eventually migrated the project to a single ESP32. Of course, that data point shouldn’t be trusted, either.)