Yes, I know it’s ancient, unsafe, etc, etc… But it’s what I have.
Will an ESP device running ESPHome connect on a router with only WEP?
Buy a more recent AP instead of using so old and unsecure devices 
You didn’t answer the question.
Nope because it’s obsolete technology but if you have an ESP with you just flash it and give it a try
If I just had an ESP or two, I would have reflashed them years ago. I have almost 50 devices. Most are installed in wall boxes or appliances. You have no idea of the enormity of the task of opening every one of them to reflash. I will try to reflash OTA first, but some of the older ones will not make the leap and will have to be manually reflashed.
Is your answer because of your bias or do you know it for a fact?
Well I know that ESPEasy (an alternative firmware for ESP) doesn’t support WEP and I just checked to be sure but ESP in past supported WEP only if explicitly requested by code so I don’t think you’ll be able to connect at WEP AP with ESPhome too 
as ESPHome doesn’t have that option.
Thanks, I had to do a fork of Tasmota to enable WEP. I think you can understand that I am trying to postpone the pain.
I have several ESP8266 devices connected to a 128bit WEP network. Running Tasmota , my own firmware, in the past Tuya firmware. So the ESP8266 is capable of it if asked.
Exactly but current firmware building tools like ESPHome, ESPEasy don’t implement that option as WEP is obsolete since quite a while now !
The question wasn’t if the ESP was capable. I have some 40-50 devices, mostly ESP, on a WEP network. As I said I fork a copy of Tasmota and add
WiFi.enableInsecureWEP();
before each occurance of
WiFi.begin()
and the device logs on to a WEP network just fine.
Why should the firmware I use force me to use WPA when it has absolutely nothing to do with the operation of the firmware? It should be my choice to use an insecure protocol.
I could probably do the same with ESPHome, but I am not a fan of Python. I will eventually start using WPA on my network even with the pain of manually re-flashing many devices.
In full agreement here. I’m sick and tired of “the powers that be” making decisions in the guise of “security” which forces me to work they way they want to work.
Honestly, I blame Google. First Chrome starts to say “Insecure” for plain HTTP sites. Then Google ranks plain HTTP sites lower. Then Google decides that TLS 1.1/1.1 is insecure so they start with a by-passable warning (which let’s face it, stops 90% from continuing). Then a full screen red page browser warning and no easy way to proceed forward.
Stuff like this has cost me tens of hours of my time and my customers tens of thousands of dollars. Because it means upgrading or proxying services just because a browser says it is insecure.
Google, et al are just covering their collective arses to protect them from stupid consumers.
Anyway, I bit the bullet and am moving all of my devices to WPA2. Two days and I have most things working. I have some cameras that need a ladder to plug in an Ethernet cable to get them to log in over WiFi again. It’s going to be in the 40’s tomorrow, so pleasant outside.
The one device that I just can’t get on WiFi again is my Epson printer.
On the rant- in my opinion Microsoft is the worst offender for “protecting” the consumer from themselves. I have three Intel NUCs running various applications and I find it easier to map drive P: to my Plex server and drive Z: to a shared folder on the same 500gB SSD. Microsoft won’t let me use the same IP address for each network map. (192.168.1.54/plex and 192.168.1.54/public).