Will Lumen be discontinued?

I don't know that there is anything official, but you can extrapolate the sentiment from certain Taylor Otwell tweets:

• https://twitter.com/taylorotwell/status/1409197614877270017
• https://twitter.com/taylorotwell/status/1212832185830510592
• https://twitter.com/taylorotwell/status/1212833250483277824
• https://twitter.com/taylorotwell/status/1306287663109091329
• https://twitter.com/taylorotwell/status/1441101127496323080

But, on the other hand, it is being maintained still - last update was earlier today (at time of writing).

If the answer is yes. Here's a quote from lumen homepage.

Have a Lumen project you want to upgrade to the full Laravel framework? It couldn't be easier. Since Lumen is powered by Laravel's components, just drop your code into a Laravel installation. You won't have to change a thing.

I think the better question is "why not switch either way?". Taylor suggests people do so over and over.