Appears that the chat at delta.com did work in the last few days, even if there was no follow-up message from the chat bot.
Just opened the chat again and discovered two things:
1) Yesterday, an agent replied just 30min after I replied with "representative". However I thought the chat wasn't working (because there was no auto-reply) and had already closed the browser window at this point.
2) The chat bot is once again working properly and replied with "Your conversation will be assigned to a specialist as soon as one becomes available" etc. However, after 5 hours and frequent prompts to reply "yes", no agent was seen so far.