Do you really think WhatsApp doesn't work on phones like Jio because WhatsApp doesnt has enough capabilitys to support it ? WhatsApp actively blocks phone models that doesn't fit in their needs. WhatsApps first priority is blocking chat-/spambots, because they are only making money with selling access for companies. To enchant the bot detection, the WhatsApp API needs to get more and more complex. If a older or cheaper phone doesn't support the complex API anymore, botblocking is more important than backwards compability. It follows that great tools like the Matrix bridge never will working with WhatsApp.

I don't know for sure why it doesn't work on Jio phones. What you said makes sense, though.

WhatsApp should be able to figure out a way to perform human user detection on these cheap phones. What type of API does FirefoxOS lack?