There's always Waydroid. Might need some tweaks to make it believe it has a real phone number attached to it, but it should work.

but in the past you needed to log in to your phone at least once a month for the web session to keep working. Is it that still the case?

Yes, unfortunately. From my limited experience trying to get it off a smartphone, leaving the number on a dumb phone and trying to activate on a sim-less smartphone, it simply won't work if doesn't detect a mobile number in its host Android

It doesn't use servers to keep messages, so when you first access from outside the "original device", it downloads the last few received messages of every conversation still in your phone, but rarely everything.

@robin@beehaw.org

Whatssapp is designed as a surveillance app, it's primary purpose is to collect data from your phone for Facebook to analyze, so they make it basically impossible to use unless it's installed on your phone.

I used waydroid for whatsapp, though any android emulator will probably work. To create the account, there was an option to verify a phone number by sending a text message (to a dumb phone with a burner sim card in my case). I use whatsapp web to read and send messages, and only have to open waydroid every few months to sign-in again.

Waydroid + Matrix bridge

You can always give a shot at using a third party client (possibly acting as bridge for other/better protocols, like e.g. slidge.im>xmpp or the buggy matrix equivalent), but you need to keep in mind that they will all require you to authenticate (and remain authenticated) using a smartphone, and that usage of 3rd party clients is forbidden from WA's terms and conditions (which may lead to your account being blocked/deleted).

I believe WhatsApp needs the mobile app to connect to WhatsApp's servers at least once every two weeks.

I think your best bet would be getting the cheapest phone you can find that will run a recent WhatsApp version, and then just leaving that at home connected to the internet. You could then use any WhatsApp web client (the website, some app, a matrix bridge, ...) to actually use WhatsApp on the go.

It's been a year or two, but last time I tried it their app worked fine on x86 Android in qemu. Not the most efficient way to run it, but at least it's isolated from the rest of the system.