Discord works fine. Whatsapp broke a few months ago for me, when my Xperia 10 II was on 4.6. However, as I had received my C2 at the time, I didn’t really investigate and just reflashed the 10 II to Android. I would not be surprised if it was something I did that messed up the Android compatibility.
Outerplane & Citymapper: I don’t know what these are so I can’t say
Firefox: works
LinkedIn: Should work. I don’t see why it wouldn’t work but I haven’t tried
Discord: Works, without notifications though
Slack: Should work. I don’t see why it wouldn’t work but I haven’t tried. Just note that you probably won’t get notifications
WhatsApp: I haven’t tried myself but from what I see here in the forum, it should work…? (without notifications though)
Workday & Chase: I don’t know what these are so I can’t say
Lloyds bank: For banking apps, it’s always hit or miss. You can look here: Banking apps on Sailfish OS
Contactless payment won’t work as far as I know because I think Android AppSupport can’t access NFC. But I might be wrong
Whatsapp works fine. Only problem for me has been when transferring backup from previous phones. If you start from scratch I do not expect any problems.