How can I get notified when a flight between from a given airport to a given country becomes available within the next n days?

Assuming you want to fly home (and you should only be trying to fly home), your best bet is the consular staff of your country. For example, various Canadian government agencies are tweeting about flights to Canada from specific airports. Here's a snapshot of the feed of travel.gc.ca:

In addition to following your country (and subsets of it like the "Canada in Lebanon" and "Canada in Singapore" accounts you see in the screenshot) you should have registered with your government so they can email, call or text you, and you should probably have called your embassy or consulate for advice about how to get home.

If you aren't trying to get home, but are trying to reach a relative or someone else who really needs you, these same sources are the ones that are likely to help you. If you think your travel need isn't urgent enough to get an embassy involved, then stop trying to travel. People are desperately trying to get home, to reunite families, to get out while they can. If you've just always wanted to see a particular place, now is not the time.

Bottom line: make sure at least one qualified human who is connected to your target country knows you're trying to get there, then start monitoring the heck out of airlines, airports, and those embassies and consulates on all the social media you can think of, and getting yourself on group chats and multi emails and facebook groups, so you'll be notified when there's a chance to go. And be ready to go when the chance comes.